Can't remember where I saw this, but they suggested using a companion angle ruler to cut the triangles. When JoAnn's had the quilting notions on sale for 50% off or I get a 50% off coupon, I buy up different types of rulers. With the companion ruler you cut your trianles from strips of fabric. On the angled sides of the ruler it will give the finished triangle base size and down the center the size of the strips to use.
So for a 4" finished square for example, the strips would be 2 1/2". Sure made things much easier than cutting a lot of squares.