the scant 1/4 inch. every quilter's nightmare.
here's the deal. it has nothing to do with the fact that the fabs are precut. it has to do with how thick the fabric and - to a lesser degree - how thick your thread is.
so - drum roll - the perfect scant may very well be different for every new project.
if you want "perfectly" sized blocks you have to make some test strips before you start working on the actual blocks. keep fiddling with your settings until your test strips come out correctly. then write down the settings so you can go back to them if you've changed them for some reason. keep the settings notes with the fabrics and pattern for the project so they don't get mixed up.
it took me 20 minutes to find the right scant for one project. blech! :thumbdown:
i almost never worry about it anymore. i just make sure to use the same allowance consistently throughout a project and live with a slightly smaller outcome.
there are only two times it really matters:
(1) when you're trying to match HSTs or Flying Geese to square or rectangular patches. you'll usually need to trim them back to match up to the squares and rectangles.
(2) if you're swapping blocks with somebody. then you still have to hope your scant and their scant are close enough to the same scant.