Regarding calculating the scale.When you go to linuxcnc website you dig into folder "motor traj" file (
INI Configuration ) there is some formulas how to calculate it. I've tried those but didn't manage to get a close number to 500. Then yes I did it the way you are guessing ( a proportion ).
But I got the harmonic drive from a guy who didn't knew the exact ratio of it and I first had to find it. I made a small sharp edge arrow that i mounted on the harmonic drive shaft via one of the 6 bolts, then scratched a small mark on the aluminium mount and did one full revolution on the shaft. Then looked at how many degrees the software did. In my case is was 496 . Then it was - x= 496*90/360 = 124. This was with the driver set at 1600 micro steps. I did a lot of testing on which microstep setting the motor was starting/working smoother and i finally figured out it was 2000. This again changes the SCALE ratio , so i calculate it again. I think by memory my final SCALE number was 861, How I test it if it's exact 360 degrees per turn: I simply tell the machine to make 6 or more turns and watch the arrow and the mark, if after all the turns the arrow stops exactly at the mark , this means the SCALE is correct and it's not accumulating any big error after a few turns,
This will be more seriously readjusted with a dial indicator measuring a flat face i do on a square stock.
This 2000 microstep setting made the motor even slower , but running smother. You can try to increase the speed of the motor from the ini file , but if the speed is above the max RPMs/ratio of the motor - the stepper stalls and the machine is going into E-stop.
Why having slow 4th axis is a problem.( if you don't have the A-axis homing kit connected ) :
1.When doing a simultaneous 4th axis milling , after let's you do a multiple dept's contour and let's say to mill this contour the axis do 4 turns , you have to watch it rotate 4 negative turns to find it's exact 0 or at whatever degrees it start the contour at every next dept.
2. It limits your cutting feedrate. For example I made my g-code with a 500mm/min feedrate and when starting the code , the machine automatically (depending you your max turning speed/scale ) dropped to 15mm/min .... you can imagine this is 33,33 times slower from what is desired, right ?
I've found info that if you have a 4th axis homing kit it rotates to the closest reference point without making turns : look what i mean
Didn't tested this yet as I don't have a proximity sensor with DIN 6 connector , but may work on this soon.
This is why a pulley will be made to make this thing turns at least 10x faster.
I can post pictures of what you are interested tomorrow.