Kitsie

Here's the math I use for any size Square in a Square block: Let's say for a 4" finished block.

large, center square = finished size of block + 1/2" = 4" + 1/2" = 4 1/2"
small corners = finished size, divide by 2 + 1/2" = 4 / 2 = 2 + 1/2" = 2 1/2"