It's weird - when I sew pieces together on the straight of grain, the units end up really close to the size I am expecting them to be.
When I sew triangles together on the bias side- using what I think is the same seam allowance, they squares end up skimpy.
I've also resorted to making the HSTs slightly oversize and trimming down.
It may waste an 1/8 inch of fabric on two sides of the square, but it sure saves a lot on aggravation.