Acoustic Guitar Construction

So I've got a question, and want to ask the smartest group of people I know to help me out!
Unfortunately, they are not available.....

So....this rosette-in-the-making is 122 mm in diameter.
I want to take it down to 120 mm in diameter.

I can think of various cumbersome and probably unsuccessful ways to do it - I'm good at that - but I'm sure there is an accurate and quick way to do it.
Any ideas?

1 On a scrap board draw a square where the sides of the square touches the rosette. Draw 2 diagonal lines corner to corner. Lightly glue the rosette on scrap board in the square. The cross is your center for a pilot hole for the router circle jig.

2 Cut a bit of scrap with your circle cutter to the inside diameter of the rosette. Use it like a 45 rpm adapter double taping or gluing the rosette with it inside and then routing off the 1 mm.

Thanks John!

Or Plan B -- make a new rosette.

(Plan B may be simpler and faster than the alternative.)

I gave John's idea a try, and it worked ok, thankfully.