When we were doing the pinwheel project a member, Ceil, came up with this formula that works!
"Whatever your block side is subtract .5 then multiply by 1.4 then subtract .5. "
Starch or sizing the fabric is a must before you cut out the squares.
So you would have:
to end with a 6" block
Start with 2 5.5 inch squares
5.5 minus .5 = 5
5 x 1.4 = 7
7 Minus .5 = 6.5 unfinished block = 6" finished
To get a 5"
Start with 2 5 inch squares
5 - .5 x 1.4 - .5 = 5.8 trim to 5.5 unfinished = 5" finished
Try it on some scrap fabric.
If your seams are a true 1/4" it should work.
HTH