I haven't seen the manual but I think you may be using the wrong parameter. Usually, Numerator/Denominator relates to gearing, it's often used in servos for electronic gearing.
So in your case, if you have no gearing then it will be 1/1. The actual pulse setting I'd expect to be in another parameter, often it's the encoder pulse parameters that dictate the output pulses per unit(mm) so it could be you need to enter 320 into the encoder parameters.? (might need to divide this by 4 if the parameter expects encoder base count as it will multiply x4 as it will assume your using quadrature encoders.!)

But without seeing the manual I could be way off.!!