Why is it 30%?
100fwkw, 30kw to parasitic loss.
Same drivetrain, 200fwkw, 60kw parasitic loss?
Doesn't make sense. There is a small increase in energy converted to heat in the transmission/diff when more power is put through it, but it's nowhere near double the loss as these low power levels, the loss does not increase linearly.
I'd suggest the loss for t50/w5* boxes and t-g series diff is closer to a constant 20kw. Dyno results for stock motors seem to suggest this.
I spoke to a dyno engineer about this subject in length as the percentage thing has always had me stumped. He agreed, for most vehicles the loss is closer to a constant 20-30kw.