Disregard the Buell method of tension:
X1 (or any 99 onwards tuber)
With the bike on its sidestand, you should be able to lift the bottom run of the belt up to touch the swingarm, whilst holding it there, you should just about be able to push the top run down to touch the swingarm.
This will result in at least 2-3'' of slack in the centre of the bottom run.
If you only have 1'' in the top run at the moment you are trashing your wheel bearings, gearbox output ballrace, the belt and the pulleys, also you're restricting rear suspension movement.
The pulleys used does make a difference, thus 2-3''
This 'rule of thumb' technique was derived by disconnecting the rear shock and lining up the axle, swingarm pivot and front pulley centres. The belt was then adjusted to be taught at this point as this is the max extension.
For those that just like to experiment, try the above (ie. disconnect the shock-frame supported ) with a tighter belt....... you will have to force the wheel up over centre, where it will hang on the belt until you force it back down.
S1 and M2 (steel swinging arm, no tensioner pulley)This also applies to the older arm, the slack should be 2" to 3" in centre of bottom run while on the sidestand. This will be the same on a paddock stand where you can spin wheel round to look for tight spots.
Regardless of the tuber model, if you've set the belt to be taut at the point of max elongation (spindle / swinging arm pivot / sprocket centre aligned), then it'll be spot on.
08 Specialized Langster