I've run out of beer reading suspension threads.
A couple observations:
1 Kg is 1 Newton. (there's more to it than that)
The spring rates must specify a weight and distance (Kg/mm, N/mm, Lb/in)
The rear spring rate of 85 must be Kg/cm. (475 Lb/in)
a 550 Lb/in spring is 98 Kg/cm
It does seem awfully stiff for a 500-550 lb bike and , say 200 lb rider. Some percentage of the weight is on the front ( anyone know how much??) and some of the weight is unsprung (wheel, tire, brakes, bevel box, and part of the swingarm and shaft).