The braking system should feel seamless, once it's dialed in. There's little need to start from a locked-wheel situation.
My method: With a Prodigy P2 controller, it will display the voltage to the trailer (notes it is working). I pull the trailer at around 20 mph and apply the manual slider to engage the trailer's brake system. If it slows the combination to a gradual stop, that is the starting point. I can then increase or decrease the trailer force by adjusting the trailer's brakes, depending on the towing situation (Interstate, city traffic, hills, etc.). I've found that once I have a sweet spot, I don't fiddle with it at all.
BTW, my trailer brakes have been rewired with a parallel wire to each axle end and brakes adjusted for proper contact.