Just a thought for those who want to end up with a particular size. Start with that measurement and add 3/4" to it. It will give you some wiggle room. If you don't want any and I mean any wiggle room only add 1/2". Example if I want a 2 1/2 inch block when I am finished I will add 3/4" = 3 1/4" Then I can square up with less than 1/4"
For the perfect people they can add just the 1/2" with no room for a wiggle. Again the 2 1/2" finished with adding 1/2" will equal 3 inches. Will have to sew a very scant 1/4" all around to have that 2 1/2" block.