Brushless FDR

[walden]

Hi All

I know this question has been asked many times

What's the best way to work out the FDR with brushless motors?

I'm running a 10.5, 1/10 4wd off road on a large 1/8 track

Spur gear is 84

Is it spur divided by pinion X internal ratio?

Can anyone suggest a starting point please?

Thanks for your help