Jim has it right, the lock-up point is just a step to establish a reference and then you adjust forslightly less from there.
Here's an extract from the Prodigy OM; note steps #3 & 4:
"1. Connect trailer to tow vehicle.
2. With engine running hold manual full left and set
Power Knob to indicate 6.0
3. Drive tow vehicle and trailer on a dry level paved
surface at 25 mph and fully apply manual knob.
✓ If trailer brakes
❑ Turn power down using power knob.
✓ If braking was not sufficient:
❑ Turn power up using power knob.
4. Repeat Step (3) until power has been set to a point
just below wheel lock up or at a sufficient force as
to achieve maximum braking power.
5. Using the brake pedal, make a few low speed
stops to check the power setting. Trailer braking is
initiated and terminated via the stoplight switch.
When the brake pedal is released, trailer braking