489 R&P, good choice?
You have to know, you can not guess at what your lowest corner rpm is and what your highest rpm is before going into the corner.
I haven’t tuned anything that turns corners for 15 years but only one guy in 25 knows what those rpm numbers are.
If your lowest rpm is 3500 and your highest is 6500 you can figure the gear ratio.
Really, the lowest rpm is for you to tune from. It doesn’t affect rear axle ratio but that number is critical
Any thing else is just a guess.
I’d say ask around where you race what gear ratio they are using but I doubt they know what the rpm is either.