I am rather sceptical about this. Not because it's a bad model overall, but because I suspect the amount of fiat coming in does not correlate well to adoption. Namely, over-represented speculators must have vastly over-pumped the market, so it's hard to tell where we 'should' be at this point.
PQ=MV, M=k*n^2, n~r*exp(t)+fCheapness is near historical maxima. That's all you really need to know, if you think BTC is technically and politically evolvable to serve as, e.g. SDRs, with probability > 0.001%.
I had intended to be levering up about now, but in the event I decided to keep it down to about 80%, so that I can short stock markets (and bull-ride the faster-cycling bubble waves on MRO, with the small amount suited to its embryonic liquidity state) while we wait for the party to start rocking. At the dogleg I will lever up, if I'm functioning. Less than 80% sacrifices too much, takes too much risk, for my present taste.
I've no doubt that impressive growth is possible. But I'm slightly cautious of equating speculators' with regular punters' money. It could easily be an order of magnitude bigger.
But there's no arguing with people who use that kind of language.
(Perhaps you could clarify the terms - I assume M = market cap, n = number of adopters, k is your constant?)