Assuming that you have a factory controller and probably your only adjustment is the gain I start by testing it at low speeds in a safe place. Set it near max and go to about 5mph, apply just the trailer brakes with the control on the brake module and see if the trailer brakes lock up. Make adjustments and repeat until they will just lock up with the brakes fully applied then drive it and verify that with normal pedal pressure you are getting expected stopping. This all needs to be done with the trailer at normal weight and you will need to fine tune it if the weight changes significantly for any reason. You may want to try a panic stop at low speeds and see if you get good brake action on the trailer. It all comes down to trial and error to get what you are comfortable with and what gives you a rapid but controlled stop. Your truck may have other settings to adjust, my Ford has a setting for the initial effort as soon as I touch the brake pedal. Again, trial and error seems to be the best solution since there are so many variables. You want to end up with the truck stopping like the trailer isn't there. In other words, the trailer is stopping itself for the most part.
