Ogy:

This one has been beaten to death time and again.

How or if the royal cubit was mathematically derived is unknown...

Divided into 28 parts, in turn, these are grouped into seven parcels of four.
When complete, the finished product is similar in length to the arm of a human being and the 7th part of 4 units each is similar in width to four finger widths of a human's hand.
Hieroglyphs were used depicting a bent arm to illustrate the cubit of 28 parts, a hand to illustrate the 1/7 part called a palm and a finger for the 1/28th part.
That is all that is known...!

Absolutely no evidence as to the first person/place/country/time/etc. that determined the standard length of the Royal cubit.

Best.
Clive