Need to calculate angles to cut capstone for a circle
I am building a 36" diameter circle shaped flowerbed for my wife. I'm building it from 4"High x 8"Wide x 5"Deep retaining wall blocks, is 3 courses high and I need to calculate the angled cuts needed for the sides of the capstones. The capstones I have are 2"High x 9"Wide x 6"Deep. Is there a formula for this calculation? Or is this gonna have to be trial and error?