So I tried my formula for the motor tuning and it seemed to solve my origanal problem (the awful noise and no movement when continuosly jogging or setting the homing speed percentage anything above 3%). Muzzer, when I altered it to your formula (with the resulting value of 416) the issues came back. Anyway changing it back to what I thought it's now homing nicely on X and Y, next job is to wire Z home proximity switch.
Just a thought though, perhaps 1600 divided by (5 x 1.3) maybe correct way to do it. Seems logical?