——>WE, as in WE people now living, do not have any idea how long Noah’s cubit happened to be.
Sure we do...
from the link I posted:
The ancient cubit measurement, which Noah used in constructing the Ark, would almost certainly have been the same as that which was used by the civilizations which arose from the descendants of the survivors of the flood.
The ancient cubit has been identified as measuring 0.5235 metres (20.6 inches), and has been found to be the unit measure which was used in the construction of the pyramids in Egypt, and in Solomon’s palace in Israel. By converting the length of this ancient cubit, into metres, the dimensions of the Ark, would be as follows:
LENGTH: 300 CUBITS = 157 METRES (515.9 FEET)
BREADTH: 50 CUBITS = 26.2 METRES (85.7 FEET)
HEIGHT: 30 CUBITS = 15.7 METRES (51.6 FEET)
It is a guess, not an actual conversion constant. A guess. No more, no less.
Well now, that wasn't Noah's cubit, was it?