How much plus or minus from the expected measurement do you consider acceptable when you make an item?

For example - if the unfinished block size is supposed to be 12.5 inches - and this was for a group project - what measurements would you consider acceptable (and easy to work with)?

(I also know that many of you are miracle workers in making almost anything/everything 'work' - but if you could have your druthers - ?)

I know that 'exact' is just about impossible to attain - so how much leeway do you allow yourself?

Even manufacturers of low tolerance items have a plus/minus from the expected measurement.

For me, my tolerance would be 12.25 to 12.75 inches if 12.50 is the target size.

Because - I could make 1/8 inch seams on the smaller one and the points would not look overly floaty, and I could fairly easily ease in the extra 0.25 inch on the larger one.