The provisioning of a Sandforce drive (taking out FW, bad blocks, and erase pool) is the baseline 0%. So it is 128,000,000,000 bytes as the baseline for a 128GB drive (same capacity as the popular 128GB indilinx 'barefoot' drives). The overprovisioning is then a simple calculation (all in the decimal number system):
OP% = (128-100)/100 = 28%
OP% = (128-120)/120 = 6.67% round up to 7%
It is not based upon the raw memory capacity which would give different numbers for both 28% and 7%, namely 37% and 14.5%, respectively.
This is how Sandforce is coming up with 100GB and 120GB drives... It's the same physical hardware with the differences being done in the provisioning that is set in the firmware version that is installed on the drives.
100GB drives have 28% over-provisioning
120GB drives have 7% over-provisioning.