I never said it was linear.
You know what the car losses through the drivetrain from factory.
Example, the old GTS HSV Monaro's. Rated at 300kw, make about 220-240kw@wheels.
Standard GTST, rated at 180 odd kw, make anywhere from 120-140kw@wheels.
So if the same GTST makes another 110kw@wheels to 250kw@wheels, we re-add that 60-40kw loss, so it pumps it up to 290-310kw@engine. However the engine is now producing more power, therefore everything is slightly increased in loss. This is the figure that can not be given by a percentage, or a fixed linear figure. It varies. I'm sure there would be a close equation you could do to work it out more accuretly. The more power you make, the higher the additional loss through friction, fluids, heat etc.
So in a bit of a maths equation.
kw@motor = kw@wheels + original factory loss (say 50kw to be a nice number) + 'unknown variable' (additional loss as power increases)
= 250kw@wheels + 50kw original loss + 'x'
= 300kw@motor + 'x'
This 'x' value is one that you have to guess logically. It aint going to be 100kw, it aint going to be .5kw. A good educated guess in this case could be 5kw. It will obviously slightly increase with the more power you are making. at 1000kw@motor it might be 50kw.
That is about as accurate as you can get with guessing, and for most people i think it is good enough to have that minium value and a little bit extra. I'd rather under qoute something than over qoute.