If the 15% is actually accurate, then 87 grains would be the equavalent of 100 grains of pellets.

87 X 0.15 = 13.05

87 + 13.05 = 100.05

But the 15% is probably just an approximation. I would trust Cayugad's "intuition" as much as the aproximation, so probably 90 grains would be about right.