If you are off by one tooth, your engine will run like crap. A good double check is to see that the middle of the intake/exhaust overlap period happens right as the piston reaches top dead center. On almost all four stroke engines, the intake valve starts to open before the exhaust valve is completely closed. The short period where they are both simultaniously open is called the "overlap" and it is an exellent reference point for engine timing and if the timing is off by even one tooth, it will be obvious.
Remember, the piston reaches the top of it's stroke in the middle of the "overlap" period.