Weighed my trailer the other day, the pin weight comes in at 12%. Had the fresh tank full which resides behind the axles. An educated guess is the pin will fluctuate from the low of 12% to maybe 17%...
520 lbs. is only 80 lbs different from manufacturers specs for pin weight. If your worried, put 80 lbs in front. If it towed fine, I wouldn't worry about it. At least you don't have to worry about overloading truck.