Set up your machine with needle, grip and tube. Then adjust your grip so that the needle tip is flush with the end of the tube, at its highest point. Then if you can move the needle with the machine off, even better, if not as slow as it can run then see how far the tip of the needle comes past the tip of the tube. Sounds complicated when writing it but it's actually simple
set your needle hang so that the needle is all the way out of the tube at all times.
measure how much needle is sticking out of the tube when it's at its highest point in the cycle.
Measure how much needle is sticking out of the tube when it's at its lowest point - fully extended.
The difference is your stroke length.
When i use my rotary pen at 8 volts, had problems with color packing
But when i use the cheap rotary, at only 5 volts skin bleeds alot. Ok when i lower it to 2.5v.
is it normal for it to run so low?