Jogging with feed per revolution (F/rev)

Hello everyone,

My name is Philipp and i started a new project a couple of weeks ago:Retrofitting a Weiler 160 Cnc to be servoconventional (like a E35).I want to keep the original DC spindle motor and replace the X and Y servos with 750W Ac ones.(got all the parts already).

I use probe basic lathe with a touchscreen and a Mesa 7i76e and a 7i85, which works already. Now to the problem:The "pendant" for the conventional part makes some trouble:

Schematic: 
first page is the "pendant" that goes on the machine saddle, second one is the control cabinet with touchscreen.

I have printed this mockup:  

The problem is the "Vorschubsrichtung/ SVH NA 4022", its a latching joystick.
I want it to feed in the selected direction, but with a defined feed per revolution. So the feed scales with the RPM of the spindle.
I have looked through all possible articles, but could not find anythig matching.
Maybe someone here has an idea on how to do that?

Regards,
Philipp

Thanks for the answer, i had feared that... Since that was too much trouble for me, i went another rout: Using a stepgen and a potientiometer, together with a scale function. Works as well, but is not really elegant.

Cheers,
Philipp