Vacuum braking value in Fitted wagons

My current crop of LYR wagons have all been "unfitted" and the wagon (xxx.wag) file contains the following lines in the braking section:-

BrakeEquipmentType ( "Handbrake, vacuum_brake" )
BrakeSystemType ( "Vacuum_piped" )
MaxBrakeForce( 0KN )
MaxHandbrakeForce( 15kN )
NumberOfHandbrakeLeverSteps( 100 )

My latest wagon however is "fitted"; i.e. has a vacuum brake. I assume that I can use the same lines but need to add a value for xx to the following specific line:-

MaxBrakeForce( xxKN )

Can anyone please tell me what is the best way to determine the value of xx?

Having gone back through some of my old reference data - probably should have done that first - I see that the MSTS documents suggest 13% of vehicle mass as a value for the MaxBrakeForce. I have also seen a reference to 20% of vehicle mass as well as 50%! Now I know that the ENG/WAG files for MSTS don't exactly match real life, but I suppose in the absence of any other guidance perhaps I should adopt 13%?