1. A lot of problems/missconceptions come about because people use the advertised figures as a baseline and assume their rear wheel kW on a dyno compared to whatever was advertised gives a % drop. The advertised figures are rarely accurate. Depending on which dyno it was run, the rear wheel figures may not be accurate. Divide one by the other and the error multiplies. Use it to compare to fwkw on another engine and the error multiplies again. Our dyno has barro, temp and humidity compensation, is calibrated regularly and corrected to SAE J1349 which is what Tickford (FPV) and Toyota use. Unless your engine has been tested on a similarly set up engine dyn, your losses are very hard to determine.
2. Loss is not by Percentage. There is no magic formula that says a white car with 17 inch wheels on a sunny day with Auto trans 5.0 and a sunroof will losse 32.675 % to the back wheels. A given gearbox/transmission will take a certain torque to turn it over at a certain speed and temperateure. Likewise for the diff, unis, bearings, tyres etc. After this, the higher the torque being transmitted introduces a higher load on components due to the helical shape of the gears in both the box and diff, ie thrust. This also effects bearings as side loads increase. The increased friction, heat and loads increase the loss to the back wheels.