I think the Z has slightly lower drivetrain loss than many other RWD cars due to the CF driveshaft and reasonably light wheels. I would have used Nissan's crank numbers and backed out the WHP/FWHP correction from that rather than the other way around.
But it looks like they're more interested in comparing dyno numbers between cars to compare relative output, and in that case assuming a fixed value makes the most sense. That lets them use larger numbers for the customers and at the same time compare dyno figures without manufacturer claims changing things.
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