documentclass[12pt]{exam}
setlengthlinefillthickness{0.6pt}
setlengthlinefillheight{.35in}
begin{document}
fillwithlines{1.25 in}
end{document}

defnumlines#1{
    ifdimdimexpr#1/linefillheight*linefillheightrelax<#1relax
        thenumexprdimexpr#1relax/linefillheightrelax
    else
        thenumexprdimexpr#1relax/linefillheight-1relax
    fi
}

documentclass{exam}

begin{document}
defnumlines#1{
    ifdimdimexpr#1/linefillheight*linefillheightrelax<#1relax
        thenumexprdimexpr#1relax/linefillheightrelax
    else
        thenumexprdimexpr#1relax/linefillheight-1relax
    fi
}


numlines{0.6in}:
fillwithlines{0.6in}

numlines{1.2in}:
fillwithlines{1.2in}

numlines{1.6in}:
fillwithlines{1.6in}

numlines{2in}:
fillwithlines{2in}
end{document}