Bridgeport Manual to CNC conversion...

[m_c]

You want to be measuring the lift in the slide (forward/backward movement on a knee), however you need to judge the best way to achieve that for each machine.

For a knee, I'd probably mount a DTI on the side of the body pushing against a feature/block mounted on the side of the knee, and try rocking the knee side to side, and pivot it up/down. You obviously don't want to be measuring the play in the knee nut/screw mounting, so you may need to try levering a few different ways to work out what will work best.