This is a fairly common problem. I have made octagonal bird house tops and slanting-side flower boxes using a BASIC program (I actually wrote!) as an "app" to do the math. That was a long time ago and I no longer have the program or the skill to write it. However, I have collected programs (better than mine) that do this math for woodworkers. An easy one to follow is from the Woodworkers Guild of Georgia, see http://www.woodworkersguildofga.org/...Calculator.htm Using this program with your 76.5 slope angle and 8 sides I get blade angle 21.8 and miter angle 5.72 (90-84.28).