Its not an exact science by any means but g1 * .5 = g7 is a way Ive read over the years to short hand and make a decent ball park guess. Some bullets it works out to a higher or lower ratio but they all seem to hover around a ~.49-.51 ratio
So in example .624*.5=.312 which actually works out to not far off from what canezach said he uses in his experience.