If it is any help, they claim 85 grains of loose T-7 powder is equal in power to 2 of the T-7 pellets. That's because the pellets are made lighter then their Pyrodex counterparts, so they provide an equal amount of power.
While the 15% rule is not cut in stone, it will be close. Although a trip to the range will do a lot more for you then we can.. Good luck and that's a great price on that powder. I'd of bought them out at that price.