I gave him what he asked for
He asked for equal slots and billet, hence the formula. Otherwise it would be easy to just pick a number of slots and divide the 360 degree circle. A number divisible into 360 is best if you are doing the task by hand with an indexer or divider, but with a CNC as many of us use, we can pick most any whole number practical and just go for it! But if you want an exact match between dashes or rectangles of inlay material and billet material, you need to do the math for every given diameter. He chose 0.850 as his diameter. I chose 12 slot as an example. There are many ways to achieve a goal.
Best wishes, Lou.