For best results you would want to invest in a dyno, but next best thing would be a motor analyzer at a bare minimum. Check your motor manufacturer for what they recommend to set motor timing based on amp draw. The value for timing is irrelevant and every brand of motor is different for peak amp draw efficiency.

I don't race 21.5T anymore so I can't give you any specifics but what I explained is how I would do it... if the motor manufacturer can't provide you a value to set the amp draw, then go on to the next brand of motor before you buy one.

I'm partial to Trinity which is one of the fastest motors at the club in my area, but there are plenty other fast brands too... good luck!