MJE
I dont think it matters when you add the sugar, it will still give you the same OG no matter what you do, as long as you hit your target volume. I used it for the first time in a mild 2 weeks ago, added it at about 5 mins, but I it was a PITA, cause I felt if I dumped it in, it may fall to the bottom and scorch, so is there any special way to add sugar to the boil? All you will do by adding the sugar for the full boil is to increase your boil gravity, thus decreasing your hop utilisation, needing more hops to get the same bitterness, so it is best left at 15 min, or later, IMHO.
All the best
Trent