Consider mounting your Gates sprockets on the inside of the spider thus shortening the lever arm against the bottom brackets which will also reduce the frame bowing tendency. The newer Gossamer cranks, produced in recent years in combination with the CDX model, typically allow the sprockets to mount on the inside while still maintaining enough chainstay clearance. As others have mentioned and alluded to, one can center the sprockets a bit on the spider (same with timing chains) by taking them loose and retightening after finding the greatest tension point, which helps greatly in striving for more uniform tension. That said, you may consider running the tension slightly on the low end. Gates provides a range of tension for single bike applications of their belt systems with further instructions to increase tension should slippage take place. Transposing that single bike instruction to the timing belt might also help. Lastly and as a final check of needed tension, push forcefully down on the left pedal while having someone view the lower part of the belt and or strum it for tension. If it goes slack, there is not enough tension. One can do this on a trainer with the brakes applied or being on the bike and leaning against a wall. The needed belt tension would vary greatly depending on rider weight, power out put, frame flex, length of the crank spindle (referring to non FSA systems since they may be different), and whether the sprockets are mounted on the inside or outside of the spider. Personally, mine ends up being a little loose, over time, and since it has never slipped, I leave it alone. YMMV