TKI makes one to one even gearing on the belt drive. There isn't much variation you can do with that. 15/18 is very close to 13/16 that you have stock. The 500 ktm motor is a torque motor and doesn't Rev out as fast or high as a 450. The gear ratio is wide in the transmission also. I wouldn't worry too much about changing it. Run it stock to start. Try flipping the gears in the secondary drive on the riot. 18/17 is stock, try it 17/18. Decide which you prefer, if somewhere in the middle seems ideal, order another 17 or get the tki belt drive.