Don't sweat it man. Most of those guys most likely just know which lines to take when going to the ramps they go to and at home. I'd put money on most not being any better than you are. Ex. At Senecaville Lake, the parking lot is really narrow. The same guys that bitched at people taking too long to back their trailer in, had to have people help them get out of their parking spot and took freaking forever.
Narrow, short, single axle trailers are a major bitch. I'd rather back a 40 ft gooseneck into a tight space than back a jet ski trailer down a ramp. Try to line it up as best as you can before backing up so that like mseebs said, you can just make really minor adjustments. Once you are basically straight you just make small movements to get as close to the sides as you can.