givio

I used the Tri-Recs rulers. I cut from four strips stacked. I used a scant seam. I felt my end result was suitable.

If yours are just slightly too small, you might consider using starch when ironing, sometimes that helps.

Or, if the size is too big or too small at the top or bottom of the seam, try re-ironing. If you are cutting from WOF strips, or anyway, from strips that are straight of grain, then the seam allowance that you'll sew on both triangle pieces are on the bias. How you iron a bias seam can make a big difference in size-- because you can stretch it one way or another, and sort of 'heat-set' it wacky with the iron.