As BPS made reference to, that 420 grain recommendation is if the bow is set at its peak draw weight of 70 lbs....70 x 6 = 420 grains. However, it should also be noted that this recommended weight is also with a 30 inch draw length in mind. If your draw length is less than 30 inches then you can get away with less than 6 grains per pound of arrow weight....going by AMO/ATA recommendations.

To make it really easy...if you are shooting the bow at 65 lbs with a 28 inch draw length then a 378 grain arrow would be nothing to worry about.