What is the best way to calibrate how much I need to bend my arm for different slopes. How do I figure out what slope is fully extended arm? Thanks.

Find a 2% putt at 10 feet, (not uphill or downhill).

Make a read with your normal arm bend and place a tee down, hit the putt and watch the ball behaviour, if it breaks more, add some arm bend, if it breaks less, lengthen your arm.

The adjustment is subtle, roughly a couple of inches either way.

A fully extended arm would be on a roughly 8 stimp green.

Regards,
Gareth