Descent speeds are generally a function of the Cost Index which will vary day to day unless the operator stipulates an SOP speed, plus VNAV or Managed descent won't necessarily control the speed to within 5 knots depending on data entered and actual winds. When given an RTA at Top of Drop to lose 10 minutes it is probably getting a little late. Move your flow control out to 300 miles or more and you might find a little more accuracy.