Base 99 is the cut off in this design as you have suggested. Base 100 is reached by changing the unit from 1 imperial foot to 0.99 of a foot or 11.88 inches as happens at SH.
Ultimately the GP is designed as 1111.1111r units x the megalithic yard where the megalithic yard is a unit of infinite dimension. In the model the megalithic yard varies from 31680 to 33000 units but 32640 is the unit that Professor Thom discovered in the British and French megalithic structures. How each column works will be gradually explained in this
A quote to think about
seeing as the cubit is an arbitrary variable, it's just about impossible to be wrong.
The first column is the Earth circumference in imperial feet.