I certainly don't know of an exact formula, but I think (someone correct me pls), that you generally reckon on losing 25% or so between the motor and wheels, usually a little more in an auto.
Like you said, every car / model is different, and I can't really see how any formula could claim to have good accuracy ( better than the above guesstimate anyways ).
Beyond this, even on the same car, different dyno's will produce wildly varying results anyway.
What are you trying to get from the formula ( I'd guess you want to know flywheel KW's after a mod ? ). Generally I'd say ( I think most ppl here would agree ), forget about the factory flywheel figure, and go with the dyno results, factory figures are good for impressing mates at the pub, but rwkw's will be what smokes that hsv at the lights :)
There is no real accurate way other than putting the motor on an engine dyno .
Some work off 30% loss but this is bullshit , If you had a 600 hp motor that means you would lose 200 hp through the drivetrain which = 149000 watts of energy , The drivetrain would melt with this amount of energy being absorbed into it .
But a simple one to use that some work off is RWHP = FWKW
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