i use EMC instead of mach3 but it should be a simple task of picking the highest speed at which it sounds good then finding an acceleration which responds well to a change in direction.
so basically it is a hit and miss affair were you have to test it after each adjustment
try a high speed first then lower it to suit
try a high low acceleration but it should be high enough that it can keep up with your speed because:
low acceleration = low deceleration = crashes