Johnathan,

Two things:

I always set MC to true average climb rate that I am willing to stop in. This is the average for the whole thermal, not just a few seconds.

My best calculation of speed to fly in lift if you do not need to climb is the MC speed for your current MC setting minus the lift strength. So if you are flying a MC of 5 and the thermal is 2 knots slow to the speed for a MC of 3.

Tim