Not sure where the 20.5mm # comes from current spec for '69-83 911 front is 282x20 w/ an effective diameter of 282mm
anyhow here are the specs
here's a measure of brake stress for a single high G stop which is the cumulative interaction of weight and speed potential, time would also also affects this but that's a lot more difficult to quantify.
While it's a laudable endeavor to get the rears to do more of the work, pre '84 911s are already close to ideal unless a lot of effort is made to set the car up for more rear brake.
'84 -89 are way too much rear biased and need a very intrusive p/v, the result is less than ideal. While A/Xer's often remove the p/v to help rotate the car, this isn't a recommended path for street or track use.
BTW this has all been previously posted numerous times