Jonathan,

I'd try the empirical approach.. decide on your required rapid speed. Setup for that. Then put a rope and pulley on the router so that movement of y lifts a weight... add some weight (stones in a bucket or something), about 4 - 5kg and see how fast it'll go without losing steps (use a DTI to measure that it returns to the same point after moves). I'd start with the 28:13 ratio as this gives a nice rapids at 4800-ish and see what you can manage with load... 3000 would be OK, 4000 would be good... then turn accel down a little and speed up... you'll soon find the sweet spot... plot a chart (speed along bottom, accel up side) and you'll get a line for a given weight... change the weight, different line. That way you can tune it for different materials as you'll know what its capabilities are.