Just a thought...you might also want to check on the other part of the formula.
As you know, the lead 7s are lighter than lead 6s (which are lighter than 5s, etc, etc).
Now that you know the 7s 'pattern' well at 40yds...might be worthwhile to make sure you know if they'll break a neck or skull at that distance.
A few sturdy tuna or soup cans (not flimsy pepsi cans of course) set up at 25/30/35/40 yards for penetation tests will tell you quickly the max yardage those lead 7's will penetrate.