Hi, 

In my opinion, there is no better or worse for calculating FDR.  Because FDR number tells you the theoretical torque and speed available with your current gear ratio.  I say theoretical because with hard compound drift tires, it would be difficult to get enough grip to hit top speeds.

I can understand why some people like using rear ratio to calculate FDR because that will yield your maximum speed/torque rating of your OVER-DRIVEN wheels.  Which is what will tell you how much speed or torque you have at the rear wheels, without any care of the front wheels, since the rears spin faster.

Personally we use CS ratio to calculate FDR.  Since CS ratio is relativity of front to rear differential speeds, that way we take both front and rear gear ratios into account.  This is how we calculate all of our cars so its the method we choose to use.

Both methods work and yield different numbers, but those are just numbers if they are not relative to anything.  The important thing is that everyone you play rc drift with does it the same way, that way there is a constant value you can use to compare between everyone.