If you look up to one of my previous posts, we have that, with the exception that subsequent shots continue to lose points.Not impossibility, just unwieldiness in use. I've always understood what you were trying to do, just underestimated the degree of persistence that you get everything you want.
But if you guys like having to track your shots in extreme detail and have a spreadsheet do math for you then go for it. I would set it up exactly as I described above, using a hit percentage modified by a weighting factor based upon expected hit percentage for targets engaged. I would factor your first round hits on that basis as well, and lump second round hits and higher together. The mathematical goal is to make all targets have a net-equal weight for hit percentage regardless of their difficulty. That way the net hit percentage is comparable across shooters no matter which targets you shot on any given day.
You're going to need a score sheet that has every single piece of steel at the range as a row, with first round hits in the first column, and all subsequent hits/misses tallied across the row each in their own column.
And we will introduce a difficulty coefficient if one target proves abnormally difficult. (800 @)#_ing yards), or too easy.