View difference between Paste ID: CgBCx0rL and qRENzEK7

 \documentclass[crop=false]{standalone}
 
 \usepackage{tikz,xstring,fp}
 \usetikzlibrary{calc,intersections}
 
 %Set dimensions
 \newcommand\secs{20}    % number of side divisions
 \newcommand\Width{10}   % width of page
 \newcommand\Height{10}  % height of page
-\newcommand\edgecut{1}  % how many rows to cut of on all edges, like OP
+\newcommand\edgecut{1}  % how many rows - 1 to cut of on all edges, like OP (1 is no rows)
 
 \FPeval\nWidth{clip(\Width+2)}
 \FPeval\nHeight{clip(\Height+2)}
 
 \usepackage[paperwidth=\nWidth cm, paperheight=\nHeight cm,margin=.5cm]{geometry}
 
 \begin{document}
 
 \thispagestyle{empty}
 
 \topskip0pt
 \vspace*{\fill}
 
 \begin{center}
 
 \begin{tikzpicture}
 %Mark side intervals
 \foreach \i in {1,...,\secs}{
   \coordinate (b\i) at (\i*\Width/\secs-\Width/\secs,0);
   \coordinate (r\i) at (\Width,\i*\Height/\secs-\Height/\secs);
   \coordinate (t\i) at (\Width-\i*\Width/\secs+\Width/\secs,\Height);
   \coordinate (l\i) at (0,\Height-\i*\Height/\secs+\Height/\secs);
 }
 %Mark coordinates
 \foreach \j in {1,...,\secs}{
   \foreach \i in {1,...,\secs}{
     \coordinate (br\i-\j) at (intersection of r\i--b1 and b\j--l1);
     \coordinate (rt\i-\j) at (intersection of t\i--r1 and r\j--b1);
     \coordinate (tl\i-\j) at (intersection of l\i--t1 and t\j--r1);
     \coordinate (lb\i-\j) at (intersection of b\i--l1 and l\j--t1);
   }
 }
 %Fill in sections
 \FPeval\nsecs{clip(\secs-1)}
 \foreach \i in {\edgecut,...,\nsecs}{
   \FPeval\resulti{clip(\i / 2)}
   \FPeval\ip{clip(\i + 1)}
   \IfInteger{\resulti}{
     \fill[fill=black] (br\i-\secs)--(rt\secs-\i)--(rt\secs-\ip)--(br\ip-\secs);
     \fill[fill=black] (rt\i-\secs)--(tl\secs-\i)--(tl\secs-\ip)--(rt\ip-\secs);
     \fill[fill=black] (tl\i-\secs)--(lb\secs-\i)--(lb\secs-\ip)--(tl\ip-\secs);
     \fill[fill=black] (lb\i-\secs)--(br\secs-\i)--(br\secs-\ip)--(lb\ip-\secs);
   }{}
   \foreach \j in {\edgecut,...,\nsecs}{
     \FPeval\resultj{clip(\j / 2)}
     \FPeval\jp{clip(\j + 1)}
     \IfInteger{\resulti}{
       \IfInteger{\resultj}{
         \foreach \k in {br,rt,tl,lb}{
           \fill[fill=black] (\k\i-\j)--(\k\i-\jp)--(\k\ip-\jp)--(\k\ip-\j);
         }
       }{}
     }{
       \IfInteger{\resultj}{}{
         \foreach \k in {br,rt,tl,lb}{
           \fill[fill=black] (\k\i-\j)--(\k\i-\jp)--(\k\ip-\jp)--(\k\ip-\j);
         }
       }
     }
   }
 }
 %Center box
 \fill[fill=black] (br\secs-\secs)--(rt\secs-\secs)--(tl\secs-\secs)--(lb\secs-\secs);
 
 %To draw whole grid:
 %\foreach \i in {1,...,\secs}{
 %  \draw (b1)--(r\i) (r1)--(t\i) (t1)--(l\i) (l1)--(b\i);
 %}
 
 %To draw just bounding box:
 \draw (0,0)--(\Width,0)--(\Width,\Height)--(0,\Height)--(0,0);
 
 \end{tikzpicture}
 
 \end{center}
 
 \vspace*{\fill}
 
 \end{document}