All I can add is the magic depth of backing in for an easy load is at times dependent on the angle of the ramp. A slight difference can help. I use the same rule of thumb, an average of the top of my fenders and a point on the bunks.
When at a new ramp, I pay attention to where the boat floats off as compared to the depth of the trailer, and use that as a small adjustment for going back to re-load.
If in a side wind or current, backing in a little less will give you a better point to help center the boat between the center rails before either driving, winching, or a combination of both. Much harder if the trailer is in too far, and the boat just floats off center.