Tolerance?
For some specifications, there is a plus-minus allowed for the size of a part.
When you are making (or trying to make) - for example - a 12.5 inch unfinished block - how much larger or smaller than the expected/targeted/hoped for size are you willing to deal with?
How much fudging/easing/stretching are you willing to do when assembling blocks into a quilt top?
Because I prefer not fighting with my blocks, I've learned that it is easier - in the long run - to have them all approximately the same size. They go together so much easier then.
For me - 12-3/8 to 12-5/8 inches would be a tolerable size range for a 12-1/2 inch (unfinished size) block.
I have also learned - that the smaller the block - such as the Dear Jane ones (Jane A. Blakely Stickle quilt) - the less wiggle room.