i know (in our minds) it should work- but it really does not- you do have to first subtract the seam allowance- then divide- then add it back in- and here's an example to show you why---
say you have a 5 1/2" (unfinished block)
divided in half = 2 3/4 {5.5 / 2 = 2.75}
when you remove the seam allowance first you have a 5" block- divided by 2 = 2 1/2" + 1/2" seam allowance- block size is 3" {5" / 2 = 2.5 + .5 = 3"} your block would not come out right if you cut 2 3/4" instead of 3"---you have to remove the seam allowance first- do the math- then add it back in like Dunster said.