“Even the common SRB is a point-to-point measurement.”
That is news to me. When I was in the gem and jewelry trade I always measured a mounted SRB diamond on the flats to give an estimate on the carat weight of the stone. If I were cutting a SRB for competition I would measure on the flats.
If I am wrong it would not be the first time it has ever happened. I would not be able to count the number of times I have been wrong in my lifetime.