John, here's what my owner's manual has to say about 3rd axis leveling:

1) Hang a plumb bob from your ceiling.

2) Nock an arrow.

3) Kneel down (so that you will be shooting uphill).

4) Draw back, and aim up toward the plumb bob and align your vertical aiming wire to the plumb bob. See where your level is.

5) Let down, and make an adjustment to your 3rd axis (the object is to try and get your wire lined up with the plumb bob and your bubble level).

6) Redraw to see how close your adjustment is.

7) Repeat steps as necessary.