There are several of these "best shift point" calculators available, I have found this one to be quick and useful.

Gear and Shift Point Calculator | GTSparkplugs

You can also check your speed rpm and and gear results in real life to verify and fine tune your accuracy. I was able to find my ratios by a simple internet search. Example:

Jaguar Car Reviews - Jaguar Pricing, Photos and Specs - CARandDRIVER

You will need to find your particular gear ratio numbers for your model, as well as know your tire and wheel sizes.

Edited: The physics behind the calculators, explained pretty well Here:
Enjoy!