I'm using 2MHz dspmc and servos with 3000rpm so my max speed is 3000 devideed by 10 (planetary gearbox 10:1) is 300rpm of the pinion . So 300rpm * 133,33mm = 39999 mm/min max speed. 39999 mm/min devided by 60 is 666,65 mm/sec. 1mm is 750,018 counts. 750,018 * 666,65 mm/sec = 499999,5 counts per second (max). If my calculations are correct I'm using 4 times lower limit than max limit of dspmc. It looks like you have done calculations correct.