At 10-15 mph, use the slider switch on your brake controller ONLY to stop your trailer. Do not push on the foot brake. It gives you an idea of how much braking power the trailer has on its own. This makes the assumption that your electric brake controller has this feature.
I have the integrated trailer brake on my Ford truck and yes it works as the more you push on the brake, the more braking electric power it sends out to the trailer. I'm on 7.5 on my controller. Try some quick stops when nobody is around from freewway speed on exit ramps. It's good to know what your rig will do in an emergency before it happens vs. after it happens.
What seems like the right amount of brake control power on dry pavement may be too much on wet pavement. You can cause the brakes to lockup on the trailer when pavement is wet. This causes trailer skid, fishtailing, and then a nasty accident in that order. It's something you have to try for yourself and see what happens on your rig. It's like winter driving. Try out the brakes and see how slippery it is. IF the trailer skids before the truck, your brake setting is too high.