Bigalt,
What you want to do is put a straight edge on top of your platter, parallel with the tonearm spindle. Then, adjust the tonearm so the spindle is perfectly parallel with the straight edge. Just use your eyes. You may have to play with the lighting on the straight edge (use a flashlight at different angles) until you can see both the top of the straight edge and top of the spindle. This will get the spindle much more parallel to the platter than any other method. Most levels, digital protractors, etc. are not accurate at all. Now, do not ever touch the adjustment screws on the post. Once the spindle is parallel with the platter it will never need to be touched. Now all you do is simply level the TT until tonearm does not drift in or out.
The problem with your method is that you do not know that your platter is perfectly level to begin with. Care to share how you are determing that?
What you want to do is put a straight edge on top of your platter, parallel with the tonearm spindle. Then, adjust the tonearm so the spindle is perfectly parallel with the straight edge. Just use your eyes. You may have to play with the lighting on the straight edge (use a flashlight at different angles) until you can see both the top of the straight edge and top of the spindle. This will get the spindle much more parallel to the platter than any other method. Most levels, digital protractors, etc. are not accurate at all. Now, do not ever touch the adjustment screws on the post. Once the spindle is parallel with the platter it will never need to be touched. Now all you do is simply level the TT until tonearm does not drift in or out.
The problem with your method is that you do not know that your platter is perfectly level to begin with. Care to share how you are determing that?