Sliding piston via a grabbable object

[topaz ivy]

how would i go about making a sliding piston that works like the throttle on a jet? its been a while since ive done vector math and forget how id get a specific distance value on an axis other that x y z

here's a pic of my set-up, the goal is to move the piston forward and backward by moving the large white ball closer to the smaller one, but it's going to have to work on all sorts of angles and directions

[jagged creek]

@full hull can probably help, I know hes made one of these before

[full hull]

I have a slider invention

You can look in that