On Grant's rules, 6- and 4-figure firing groups

Yesterday I wrote:
if I ever want to play Grant's rule, I need to do some statistical work to transition from a 6-figure to a 4-figure firing group.
Well, courtesy of an unusually slow commute, today I did some statistical work, and I came to the conclusion that... IT DOESN'T MAKE ANY DIFFERENCE!
If my math is right, moving from a 6-figure to a 4-figure firing group reduces firepower by one-third; assuming that hits are directly proportional to firepower, the problem is easily fixed by rolling a 6d to assess casualties, and consider in turn 5 and 6 (remember: against a 4-figure base) as a miss. Which will reduce losses by one-third! In other words: in Grant, you first roll to decide how many hits, and then you check which figures were actually hit; with my fix above, you first roll to decide how many hits, and then you check which figures were actually hit with the additional possibility that a 5 or 6 on a d6 will result in no hits on a 4-figure base.
Bottom line: one less argument against the NB basing scheme.