Hi Martin,
No I don't at this time, I think it would require a fair amount of detailed knowledge of the parameters of the ICs for any meaningful calculations.
All I can say is that the values used that I am aware of, range as follows:
C210 0.2 to 0.68uF (or higher)
R208 22K to 220K
R209 97K to 330K (or higher)

My friend in Melb. only changed C210 to 0.68uF and 33K in // with R208
effectively making it 28.7K as above..
He said it did seem to make a difference when using my Platter.

DC in his mods, placed a 33k in // with R208 effectively making it 28.7K
and he increased C210 to 0.69uF by placing a 0.47uF in //
Also R209 was replaced with 100K.

When fitting the Bearing and Base Plate to the SL1200 used as a cutting lathe, I was asked to fit a 150K in // with R209 effectively making it 103K.
This single mod is reccomended by the German supplier of the cutting attachment. and apparently does improve the torque when heavy cutting.

If you study the circuit block diagram, you will see that R209 feeds from the phase control output into the RC network, where it feeds into the OP amp, reducing this value would appear to increase the input signal across the resistive devider network.
C210 would obviously provide the capaitive smoothing /delay to any sudden changes of input to the OP amp.

However here is the caveat: if you study the diagram carefully you will see that firstly the input ECR (pin 17) of the OP amp appears to be earthed, but then it feeds into the motor control chip pin 5 as signal EC. ??
ECR (pin6) from AN6675 is shown as originating from the resistance divider network
R207,R206, R208

I think it was Paul H who first indicated to me that the diagram contained some errors.

What would probably work is for a few people to who may wish to try this, to interchange values they have tried.