Kelsey Hayes was the standard back in the day. My parents had one installed on our '72 Pontiac Safari wagon for towing the horse trailer. There was a variable resistor under the hood. It had a big wing nut and the contact between two wound resistors could slide back and forth. One position it basically shorted straight across (zero resistance, max braking) and the other direction the current would have to flow all the way through both (max resistance, almost no electric assist). The idea was you balanced the amount of braking for the load/rig.
So much for the history lesson. Modern electronic controllers do this with various degrees of magic. I agree, start a thread titled set-up of electronic brake controller. On ours you just turn a knob and it gives you a digital read out of a number [more history, on the Kelsey Hayes there was also a knob you turn on the brake controller inside the car that determined how soon in the throw of the brake pedal the electric brakes engaged]. If you have the manual (or look on line) you should be able to see what the adjustments are. Some are so sophisticated they work it all out on their own.