I just ran the calculations myself. At the seeming current ratios of 15k Pebbles and 5 gems to a moonstones, 457k pebbles would be equivalent to 30.46 moonstones, for a total of 152 gems. Since .46 of a moonstone is nonsensical, I'm down to drop that to 150 just for the sake of the math.
OK. I have 95 Moonstones and 900k pebbles right now, so at 15k Pebbles: 1 MS : 5 FR gems, that'd be 475 gems for the moonstones and 300 gems for the pebbles for a total of 775.