Jog the tool to around 8mm from the work, then slowly jog it away until the shaft of a 10mm end mill will just slide between the tool and the work. Then you know you are at the 10mm position. This avoids chipping or breaking the tools and works for both mills and lathes.

But the correct way to look at this is that you need to buy an electronic probe, as it will transform your work accuracy. It's just the excuse you needed!

I never use the handles on my mills or lathe now. The MPGs give much more flexibility and control. I used to have handles on the Bridgeport and Bantam conversions, thinking it would be useful for manual operation but ended up removing them, as it improves the response of the servos.