Each make/model is different.
The rolling roads that do not do a run down to work out transmission losses just use a calculation based on a known vehicle.
i.e they have transmission loss data on a representative subaru or evo and just apply the same maths to the next subaru or evo that runs on the rollers.
i would dig out some old graphs but gave them away with the car.
http://www.lancerregister.com/showth...astdown+dastek
If you can find some EVO RR plots showing ATW and ATF fromthe same car on the same run, you should be able to calculate the appeox losses.
the above link talks about 24% i.e multiply ATW x 0.76 and that this gave correct figures for stock cars.