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I am not aware of any mathematical formula for adjusting brake controllers. The answer lies in setting the controller high enough that the trailer brakes work effectively, but not so high that the trailer brakes lock up.
I have a Prodigy brake controller, and my particular model has 4 braking modes. I find this is very helpful because when I am maneuvering in the campground, I use one mode, and then I use a different mode out on the highway, and I use still a different mode when in the mountains. If I don't switch the modes I either get undesired lock-ups, or not enough braking assistance from the trailer. My point being, I am adjusting the setting of the brake controller all the time, and it is never set at any one setting on any given trip.
2008 Surveyor SV-291
2003 Ford 4x4 Expedition
Reese SC WD System 800/10,000 lbs
1976 Cessna 177RG
Wife, son, twin daughters, and mother