What's a good rule-of-thumb for estimating Rear Wheel HP, given an advertised HP rating for an engine? I've read articles suggesting drivetrain losses range from 15-35%. Others figure the loss as a fixed amount of horsepower, i.e. 50-100 HP. This method would imply that I'm giving up a "fixed" amount of power regardless of the output of the engine. Additionaly... is it fair to assume an automatic with a loose converter is going to have greater losses than a manual car?

I'm trying to figure what to tell my engine builder in order to develop 400 HP to rear wheels.