Edit: Posted this before reading of the accident, so depreciate it accordingly. Not important at all right now.
I can't say this from any specific drive train knowledge, but more from physical reasoning.
I suspect the relationship between shaft power and wheel power has a little more complexity than a simple proportional ratio.
Looks like y = mx.
So - skeptical of simply using the 30% thing to back calculate shaft power, like
@scottryana says.
The next step up in any mathematical model would be a classical linear relationship.
2 components, like the classic line equation in algebra class : y = mx + b.
x = measured wheel power, the input data.
b = a constant loss factor through the drivetrain - things spinning, bearings, seals, fluids, etc.
m = a proportional pressure dependent loss factor - things press into each other harder with higher power.
y = calculated input power.
And all that would be RPM dependent, which is more likely a squared factor.
You
could put together a decent model after taking enough data, but it's probably beyond the scope here.
And anyway, it's not really useful. Just armchair bragging stuff, right?