catcode`@=11

begingroup
  catcode`P=12
  catcode`T=12
  lowercase{
    defx{defrem@pt##1.##2PT{##1ifnum##2>z@.##2fi}}}
  expandafterendgroupx
defstrip@pt{expandafterrem@ptthe}


defsetdimenzerotofontheightanddepth#1#2{
dimen0=fontcharhtfont`#1
advancedimen0 by fontchardpfont`#1
ifx#2
else
  advancedimen0 by fontcharhtfont`#2
  advancedimen0 by fontchardpfont`#2
fi
}

setdimenzerotofontheightanddepth fg
edefdimzeroamt{strip@ptdimen0}

Dimen0 is: thedimen0, Baselineskip is: thebaselineskip

dimen1=baselineskip
Unfortunately it also shows the dimzeroamount when I do the calculation: dividedimen1 by dimzeroamt

The ratio is: strip@ptdimen1

The real (manually calculated ratio) is $0.90947dots$
catcode`@=12

bye

documentclass{article}

usepackage{graphics}

makeatletter

begin{document}


defsetdimenzerotofontheightanddepth#1#2{
dimen8=fontcharhtfont`#1
advancedimen8 by fontchardpfont`#1
ifx#2
else
  advancedimen8 by fontcharhtfont`#2
  advancedimen8 by fontchardpfont`#2
fi
}

setdimenzerotofontheightanddepth fg

Dimen8 is: thedimen8, Baselineskip is: thebaselineskip


Gscale@divtmp baselineskip{dimen8}

gives tmp

The real (manually calculated ratio) is $0.90947dots$



end{document}

dividedimen0 by <number>

input expl3-generic

ExplSyntaxOn
cs_new_protected:Nn ioiooiioio_getfactor:Nnnn
 {% #1 is a control sequence, #2 the dimension, #3 and #4 two characters
  tl_set:Nx #1
   {
    fp_eval:n
     {
      dim_to_fp:n { #2 } /
      dim_to_fp:n
       {
        fontcharhtfont`#3 + fontcharhtfont`#4
        +
        fontchardpfont`#3 + fontchardpfont`#4
       }
     }
   }
 }
cs_set_eq:NN getfactor ioiooiioio_getfactor:Nnnn
ExplSyntaxOff

getfactortest{baselineskip}{f}{g}

test

bye

dim_max:nn { fontcharhtfont`#3 } { fontcharhtfont`#4 }
       +
       dim_max:nn { fontchardpfont`#3 } { fontchardpfont`#4 }

catcode`@=11

begingroup
  catcode`P=12
  catcode`T=12
  lowercase{
    defx{defrem@pt##1.##2PT{##1ifnum##2>z@.##2fi}}}
  expandafterendgroupx
defstrip@pt{expandafterrem@ptthe}


defsetdimenzerotofontheightanddepth#1#2{
dimen8=fontcharhtfont`#1
advancedimen8 by fontchardpfont`#1
ifx#2
else
  advancedimen8 by fontcharhtfont`#2
  advancedimen8 by fontchardpfont`#2
fi
}

setdimenzerotofontheightanddepth fg%
edefdimzeroamt{strip@ptdimen8 %
}%

dimen0=baselineskip%
count0=65536
  loop%
  ifdimdimen0<8192p@%
    dimen0=2dimen0%
    dividecount0 by 2 %
  repeat
dividedimen8count0
dividedimen0dimen8
strip@ptdimen0
%edefx{strip@ptdimen0}

The real (manually calculated ratio) is $0.90947dots$%
catcode`@=12

bye

2^16 * numberdividend / numberdivisor

newdimenfnheight
fnheight=13.19443pt  baselineskip=12pt

{lccode`?=`p lccode`!=`t  lowercase{gdefignorept#1?!{#1}}}

expandafterignoreptthe
   dimexprthenumexpr 65536*numberbaselineskip/numberfnheight sprelax

input apnum

newdimenfnheight
fnheight=13.19443pt  baselineskip=12pt

evaldefOUT{numberbaselineskip / numberfnheight}

OUT  % OUT=.90947485284083681234 because apFRAC=20 by default.

bye

makeatletter
newcommandSetToRatio[3]{% sets #1 to be the ratio #2/#3, where #2 and #3
    % are lengths (registers or expressions).
    % The ratio #2/#3 should evaluate to less than 16384 in absolute value to
    % avoid arithmetic overflow. It will be computed as fixed point
    % number with about 4 or 5 digits after decimal mark.
    edef #1%
        {strip@ptdimexpr
         numexprdimexpr#2relax*65536/dimexpr#3relaxrelax sprelax}%
}
makeatother

SetToRatio{foo}{20000sp}{1sp}

% j'avais fait cela dans https://tex.stackexchange.com/a/328894/4686

% Octobre 2016

documentclass{article}

makeatletter

countdefddf@cnta=z@
countdefddf@cntb=tw@
countdefddf@cntc=4
countdefddf@cntd=6

% % if used with Plain TeX, un-comment this
% % LaTeX loop or any loop allowing elserepeat:
%
% longdefloop #1repeat{%
%     def iterate {#1relax expandafter iterate fi}iterate
%     let iterate relax }

% defdivdef #1#2#3{% if using plain tex

newcommanddivdef [3]{%
  % description:
  % computes R = #2/#3 as nearest multiple of 1/65536 (ties go to even)
  % then define the macro #1 to be the decimal expansion of this up to five digits
  % after decimal mark.
  begingroup
  dimen@ #3relax   % denominator
  dimen@ii #2relax % numerator
  ifdimdimen@<z@
    dimen@-dimen@
    dimen@ii-dimen@ii
  fi
  ifdimdimen@ii<z@
    defddf@sgn{-}dimen@ii-dimen@ii
  else
    letddf@sgnempty % no @empty in Plain !
  fi
  ddf@cntadimen@   % non negative denominator (we hope non zero...)
  ddf@cntbdimen@ii % non negative numerator
  divideddf@cntbddf@cnta % integer part of ratio, will be stored in ddf@cntd
  ddf@cntdddf@cntb
  multiplyddf@cntb-ddf@cnta % no overflow possible because TeX's division truncates
  advanceddf@cntbdimen@ii   % now numerator in ddf@cntb is < denom
  count@z@ % will store fractional part as a multiple of sp's
  ifnumddf@cntb>z@
   ifnumddf@cnta>32768relax
    ddf@cnta65536relax
    loop
      ddf@cntcdimen@ % denominator
      advanceddf@cntc-ddf@cntb
      ifnumddf@cntb<ddf@cntc
        divideddf@cntatw@
        advanceddf@cntbddf@cntb
      else
        ifnumddf@cntb=ddf@cntc
          divideddf@cntatw@
          advancecount@ddf@cnta
          ddf@cntaz@ % abort the loop here
        else
          advancecount@ddf@cnta % not same order as in previous branch!
          divideddf@cntatw@
          ddf@cnta-ddf@cnta
          advanceddf@cntcddf@cntc
          ddf@cntbddf@cntc
        fi
      fi
    ifnumddf@cnta=z@else % signed quantity: can not do if foo>z@ ...
    repeat
    % it is possible here that count@ is 65536
    % in case of a tie at the last unit the rounding was to even!
   else
    % here denom <= 2^15=32768 (=0.5pt), hence 65536num <= 2^31 - 65536
    multiplyddf@cntb65536relax
    % extra steps to do rounding
    ddf@cntcddf@cnta
    divideddf@cntctw@
    advanceddf@cntbddf@cntc % no overflow possible
    ddf@cntcddf@cntb % need to keep copy for later branch
    divideddf@cntcddf@cnta
    count@ddf@cntc
    ifoddddf@cnta
      % odd denom, no tie possible
    else
      multiplyddf@cntcddf@cnta
      ifnumddf@cntb=ddf@cntc
        % implement "ties go to even", the rounding was "up"
        ifoddcount@advancecount@m@nefi
      fi
    fi
    % to get count@ 65536 we would need to have N/D >= 65535.5/65536
    % i.e. N/D >= 1 - 1/131072, but N/D<= 1 - 1/D, D<=32768, hence
    % despite the rounding this branch always produces count@ < 65536.
   fi
  fi
  dimen@count@ sprelax
  % (thecount@) % debug check
  expandafterdivdef@endthedimen@ #1%
}
begingroup
catcode`P 12
catcode`T 12
lowercase{gdefdivdef@end #1.#2PT}#3{%
  advanceddf@cntd #1relax % almost always #1=0
  ifnum#2>z@
    edefx{endgroupedefnoexpand#3{ddf@sgntheddf@cntd.#2}}%
  else
    ddf@cntdddf@sgnddf@cntd
    edefx{endgroupedefnoexpand#3{theddf@cntd}}%
  fi
  x
}endgroup

begin{document}

ttfamily

divdefFOO{20000sp}{1sp}

meaningFOO

divdefFOO{355pt}{113pt}

meaningFOO

divdefFOO{1pt}{7pt}

meaningFOO

divdefFOO{10000pt}{7pt}

meaningFOO

divdefFOO{1000000000sp}{7sp}

meaningFOO

end{document}