If you can control the crush on the LHBS mill that would be a good start...
Don't be that guy that asks the LHBS to change the gap setting. Shops normally err on the side of a wide setting so customers don't complain about slow runoff / astringency.
Make sure you have your own process lined out before taking it up with the shop. Remember, it doesn't matter what your efficiency is as long as it's consistent.
Runoff rate, water/grist ratio, and pH all affect efficiency and need to be consistent. If you're batch sparging, runoff rate is especially important.
As long as you monitor and adjust pH, your water supply should not affect efficiency.
As your dialing it in, I prefer to overshoot my target pre-boil gravity and adjust with water. I just have to make sure I also adjust hopping rate and spicing, as well as my boil-off calcs. More work, but I don't have to worry about my malt flavor profile, color, or fermentability changing.
Its a bit easier to undershoot your gravity, you just have to add DME. As long as you get your target volume, no other adjustments are necessary.