As others have already noted, the brackets are not a good measure. The axle is able to move without moving the bracket. Use a ruler and take small steps.
1. With axle nut loose, loosen both adjusting nuts on the back of swingarm, so the axle is allowed to slide forward a good bit.
2. Snug up axle nut, but not tight enough so the axle cannot slide.
3. Use adjusting nuts on back to pull the axle back so chain is slightly looser than spec.
4. Tighten up axle nut a bit more.
5. Focus on chain side to get proper tension. But, every time you adjust chain side, measure distance on chain side and make the right side match. Use small steps, because if you overshoot and move the axle too far back it throws things off when you knock it forward.
6. Torque axle nut to spec.
Hope that made sense... it did in my head.