g64 p 0.001
this means it will try to maintain velocity while staying within 0.001" of any end point. thi is how alot of controls do it.
with mach3 you just enter g64 with no qualifiers. BUT you do have a global distance tolerance, as well and an angle threshold in one of the setup pages. the angle threshold turns g64 OFF, and the distance threshold behaves the same as the p qualifier in linuxcnc. the REASON its global is so you dont have to manually enter these things in your code, especially if using a very simplistic cam package. just leave G64 on all the time and for the most part, it all just works.