Right now I can't think of the formula for cutting the triangular pieces on an on point quilt, however, I know from experience to make them bigger, it's always easier to trim away than for the triangles to be too small. You need to remember that the side triangles need to have straight of grain on the outside of the quilt. If you have bias edges on the side of t he quilt it will create problems. Now that I've given you a lot of useless information, I look forward to seeing your quilt. I'm sure some of the other quilters on the QB will have lots of good information for you.