[mindcrime]

b) it's still just a guess - normalising my guess and you're guess is hard

True, but that's where the calibrated probability assessment stuff comes in. If you can at least establish obviously correct (if very course grained) upper and lower bounds for estimates and then gradually shrink them in until you aren't confident doing so anymore, you have something at least slightly better than a complete guess. And if you can choose the correct distribution that the actual values would vary over, then you can use Hubbard's approach using a Monte Carlo simulation, and get some insight into likely outcomes.

No, it's not a perfect approach, but it gives you a little something to hang your hat on.