Converting coefficient of friction to NL2 friction

Hi everyone. Let's say I have a length of straight track tilted downwards at an angle of 1 degree. I can work out what the coefficient of friction would be from the slope, but how do I convert that number into the correct NL2 constant friction parameter value? My previous efforts have involved hours of experimentation. Surely there's a more precise approach I can take?

Depends on the surface of the wheel.
Is the trackrail smooth ? or heavy pitted and poorly maintaned ?

Here's how the friction coefficient works:

Example:
If we have a 0.03m/m coefficient, it means, the track goes 0.03m down each meter. Which is equal to a slope of about 1.7°. This slope represents the angle at which the train/car doesn't gain or loose speed.