Deliberate, confused why you're even pursuing this thing about your nice fitting damaged helmet. Maybe because you're proceeding under some false assumptions:
First, doubt any of this is neatly linear or cummulative. All evidence suggests that the first fracture/crushing of the EPS will significantly and seriously reduce its ability to absorb energy. All evidence also suggests that the fractures in the EPS can be tiny and impossible to detect without doing cross sections of the helmet. After that, I'd guess each successive blow (if you're, ah, challenged enough to still be wearing it) will result in more reduction, but the first time will be the major loss. Think of it as a steep line from pristine to impact1, then I1 to I2,3, etc. segments that are shallower. Or a cool curvilinear function our engineer friends could model if they had the data.
And yes, obviously you could envision a situation where the first time is a tap, with light damage, while the second time you hit a tree, and and yes, obviously wearing a damaged helmet is better than nothing. OTOH, if you're a lawyer, you probably make enough money to afford a new lid, and you could probably buy the same brand and still love the fit. Or better, why don't you have some high tech imaging done to see how bad the damage is? Bet you could find out for less than $5,000, and that's certainly worth it for the good fit...
Second, everything I've read about concussions (see my thread on this) indicates that beyond a generality that many are usually worse than one, it's impossible to assume which time will be the deal breaker. Not only non-linear, but probably not amenable to any mathematical model.
So why are you even still asking this question?