I don't know about the 1250, but on the 1200, to go up another tooth in the front requires minor trimming to provide chain clearance.
You'll probably find the odometer is pretty accurate (at least they are on the electronic speedos of the earlier 600/1200)... so this error is intentionally built-in.
If having the speedo accurate is more important to you than the odo and you can't go up a tooth up front, you could always go down 3 teeth in the rear or look into a speedo corrector, like the
Yellow Box.