For squares cut into triangles - I have finally learned to always cut the original square a bit larger than the theoretical required size.
I have no idea of why my units always end up skimpy when I cut the starting square the theoretical size. Even when I narrow the seam allowance to a scant 3/16 instead of a 1/4 inch - the units end up skimpy.
So - I would much rather trim a tiny bit from the unit than have to pitch the whole unit because it was too small.
If I cut triangles individually, they usually work up to the desired size.
I think part of that is - when I cut with a ruler, the piece is a tiny bit larger than the "nominal" size.
Example: I am cutting what I call a 2.5 inch square with a ruler. It's actually a tiny bit larger than 2.5 inches - maybe only a thread width or so, but it does make a difference.