First, establish that the wheel is bolted into the dropouts correctly by doing the following:
1. With bicycle wheels on the ground and the weight of the bicycle supported on the tires, loosen the axle nut on both sides.
2. Put weight on the rack to push the axle into the top of the drop-out slot. It is easiest to have one person hold the bicycle perpendicular to the ground (not supported by the kickstand because that is at an angle) and push down on the rack while a second person tightens the axle nuts. If you are doing this solo, straddle the rack backwards and while sitting on the rack reach down to tighten the axle nuts.
2. Tighten the axle nut on both sides. This establishes a baseline axle position that will make tension adjustments at the sliding dropouts a one-time thing and also easy to repeat if necessary.
At this point, you very well may have a wheel that is straight in the frame and also a belt with proper tension. The belt should have about 1/2-5/8 inch of deflection in the middle when you push on it with one finger. If the tension is off, the following procedure is used to adjust the tension.
Belt tension adjustment
1. Loosen 2 sliding drop-out locking bolts on both sides using a 5mm allen wrench.
2. Loosen 10mm nut on tension bolts on both sides.
3. Check belt tension.
4. To tighten the belt, you will need to turn the tension bolts clockwise. Turning them counterclockwise will allow the belt to loosen, but you may have to push the wheel forward manually. Be sure to turn the bolts on both sides the same amount, so that the wheel remains straight in the frame.
5. Recheck belt tension after adjusting both bolts.
6. Repeat as needed.
7. Once the proper tension is found, tighten the sliding drop-out locking bolts to secure the drop-outs to the frame.
8. Tighten the 10mm nut on the tension bolt to lock it down.
Now each time the rear wheel is removed for any service, the belt tension will be easy to get right and remain unchanged. Every time the wheel comes off the tension can easily be set correctly when the wheel is installed again by ensuring the axle is at the top of the slot in the drop-out before locking the axle nuts down.