Machining both ends (depth stop)?

Hey there,

The solution must be so simple I can't see it, so I'm interested in
how the group steers me on this question.

I'm machining a part with two ends. Both ends need to be machined. I
will hold the part in ER25 collet, machine one side and flip it
around. How do I know the depth in the collet when I've flipped it
around?

Is there a trick or another tool to help me out? I'd like to be able
to make multiple units efficiently.

Thanks,

Ken