Router Circle Jig Calculator

I recently gifted myself a Bosch 1617EVSPK router, and the first project is mounting it to a old Craftsman 171.253512 router table that came with the used contractor table saw I bought myself for my 37th birthday.

An adapter plate is needed. None of the threaded mounting holes on the fixed router base line up with holes in the table. A friend advised me to cut the plate out of metal or acrylic rather than wood. I may do that. First, I’m making a prototype out of 1/2 inch birch plywood.

The challenge is that a hole is needed in the center to line up with the hole in the center of the router table. The hole is 2 and 7/8 inches. The first thing you learn when you get a router is that you need a guide for every cut. To cut a hole, you need a bigger hole, or circle jig, in which to run the tool.

How Big of a Hole do I need to cut a Hole?

What size circle jig is needed to cut a 2 and 7/8″ circle? Well, what size is the router base plate, and what size is the bit? The answer for me was 9 inches. I made a circle jig calculator spreadsheet to help you calculate your own answer: https://docs.google.com/spreadsheets/d/1j-HHztDr0gRN3G3gc9z6VhBRuo2sOvzG_dBccMmtVgg/edit?usp=sharing

Is this silly? Is there an easier way to do this? Please, let me know.