When the system is 100% functional and in good repair you should be able to lock them up which is 100% braking force.
On my current trailer I spent almost 2 years figuring out why I wasn't getting full braking from the trailer brakes. After cleaning adjusting, checking connections, voltage and amp draw as well as resistance checks everything was showing OK. As a last resort I changed out all my brake magnets and after 20 miles of burnishing them in the trailer finally was able to lock up the brakes manually. Obviously you don't set them so they lock up, but you want to be sure you are getting full brake application. I would say I was originally getting about 40%!!! And when I found this my trailer was 2 years old with about 1500 miles on it. It would stop but not as well as my previous smaller trailer. After I fixed it the heavier trailer stopped faster than my lighter one did. And it still brakes very well to this day.