The M4 with onboard esc's uses a voltage to thrust relationship and has no need for RPM. In doing so it automatically compensates for battery voltage change and the internal esc code apply's some correction to compensate for the slower response of the brush motor.
The attached xcel file should give you an idea as to what is needed to get a good A1,A2. for the -20 motor. Numbers for a 0820-15 with ladybird prop are attached as a reference. If I was in a hurry I would just raise A1's value by the percentage of thrust that the -20 motor puts out more or less than the -15 motor and you'd probably be close enough leaving A2 alone.
A good test to see how well you've done would be to put the copter on a thrust stand with a variable supply. Set half throttle with 3.5 volts applied and note the thrust. Now raise and lower the voltage to see it the thrust stay's close to what you have set.
Cheers
Steve
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