If you have a certain sized block and your patterns has four equal blocks up the side and across, then you would divide 18" by 4 and be sure to add the seam allowances. If it is 5 equal squares up the side and across the block, you would divide by 5 and add the seam allowances. The same way with six equal squares or what ever number
If I have a nine patch and I want a 6 inch finished square, I know I have 3 squares going up and 3 across. So each square is 2" finished and I would have to cute 2 1/2 because of seam allowances. If I want to go up to a 9 inch square. 9 divided by 3 squares is 3 then add the seam allowances. It works the same for any block.
You always design with the finished sizes and add the seam allowances.
If I want to find the size of the half square triangles in a square, I make the finished size of the square out of cardboard. Cut it in half and add seam allowances on all three sides. Then I find the right line to cut on my ruler by putting my cardboard on my ruler, the one that had the seam allowances added on all three sides.
Way back they folded paper to find the correct sizes..