where did you get that it is arbitrary?
and where did you do your degree?
and should you give it back?...
I get "arbitrary" from 30+ years in the business. As I said, some engines run at (certified) 118% indicated, meaning that 100% is a low-end cruise or holding pattern value. There's NO GOOD REASON to calibrate the tachometer that way, is there? I think some marketing director sat bolt upright in bed @ 0300 with that brilliant idea. Believe me, there's little sound engineering in THAT one.
But in the end, does it make any difference? Instead of % speed, or (in the case of piston engines) RPM, they could have picked radians per millisecond.
The numbers merely have to reflect the units chosen.
Of course, whatever speed the engine likes to run, the mfr. has to demonstrate overspeed tests, blade-out, bird ingestion etc. using the takeoff speed as reference, and conduct low-cycle fatigue tests and analysis on the same basis.
I beg you to put up hard evidence to the contrary!