The oem oil level (fully compressed) for the Ninja 250 is 108 +/- 2mm and for the 300 it is 100 +/- 2 mm with 10w oil.
Wilbers recommends for a 37 mm fork: 140 mm = soft / 125 mm = medium / 110 mm = stiff.
At least you have to find the best value by yourself.
Good luck
