Hey there, it seems to me that the only way to properly answer your question is to point out the fact that the passing success percentage should not matter in your tweaking. When you look at it, it's a pretty irrelevant and intangible number for several reasons.
1- In the NHL, if people were to keep track of this percentage, there would be a lot of variations because of the different game paces that you find in this league, and also because every game feels different in this portion of the game (puck distribution).
2- What's a pass anyway? Sometimes it looks like a dump-in, sometimes it's going along the boards, it might even be a drop pass.
When I play NHL 2k10 ranked I usually have a pass success percentage between 45 and 55. I play with 0 pass aim assist so a lot of the passes I do that are accounted as failures are in fact bank passes, passes along the boards, dump-ins (with the pass button, very effective in certain situation) and "puck shoving" just to get it moving forward.
So, in the NHL as well as in 2K10, pass success percentage is hard to define and also misleading. I don't know if I got you convinced of this, but I do think that a serious tweaking plan should leave aside this statistic (as well as other stats, like hits and save percentage). Thanks for reading!