Yep, trial and error. I've been towing a bunch of different trailers with my Ram and it's interesting how much variation there is for the best setting. Just lots of moving parts (weight, size/condition/ability of trailer brakes, etc.). I was doing some gravel runs recently going between an empty dump trailer (about 2K) and full of gravel (about 10K) and I was going between 2 and 6.