yep the upper fence for the vertical scale is derived from the vote totals statistics. If the set of vote totals is now in votes per square mile, then there would likely be a different upper fence. Might be worth the number crunching to figure out what that number is.
BTW just to double-check, I'm assuming that in this given fillOpacity formula, that a fillOpacity > 1 just resolves to 1. So like fillOpacity : min(votesPerSquareMile/upperFence, 1).