It doesn't really matter which one you experiment with first. You will end up at the same place when you find your load.
IME, the optimum seating depth for a given bullet remains constant regardless of the powder used or final charge weight. If you plan to try multiple powders, it may make sense for you to establish optimum seating depth with your chosen bullet first.
Normally, I start by finding the distance to the rifling with a given bullet. Then, load 5 rounds touching the rifling, 5 rounds .040 off rifling, 5 rounds .080 off rifling, and 5 rounds .120 off rifling. Fine tune from there, if necessary. (This is all based on recommendations given on the Berger website.)
Given that you have decided to restrict yourself to mag length, you may need to improvise a bit. Using a starting load with your chosen bullet, treat mag length as the max "touching the rifling" length and shorten the OAL in .040 increments (go smaller if you feel the need) until you have four separate groups to test. At least one of the groups will shoot markedly better than the others. Do your powder workup from there.
Keep in mind that bullet design may be the most important factor in your success given that you are restricting yourself to mag length. Bullets with secant ogives (like VLD's) may or may not work well for you. I have gotten VLD's to shoot with a lot of jump, but have never tried them with .160+ of jump.
I can say that bullets like the Berger Hybrids truly are insensitive to seating depth. I have also had good luck with tangent ogive bullets like Sierra Matchkings when restricting myself to mag length.
Once you have found your seating depth, try this:
OCW Overview - Dan Newberry's OCW Load Development System
The above linked instructions work just fine before you find your seating depth, too. It doesn't really matter which one you do first. I have found that when I combine the seating depth instructions from the Berger website with the OCW method, I am usually able to find my best load in 50 rounds or less.