Your formula is not consistent and probably would be impossible to be consistent. Excitebike is DL only and is different then FE. So the ratings/buyers will be different. An earlier FE would be a better comparison if possible. Anyways my guess is 50k-90k max DL for FE, but who knows you could be completely right or it could be more.
It doesn't matter if Excitebike is DL only and FE is not.
Since Excitebike is DL only, that means EVERY review is a game downloaded on eshop.
Thus since it has 27k reviews. That means ANY game higher on list has AT LEAST 27k downloads. Fire Emblem is higher on list so it has 27k downloads minimum.
Now no idea how many people review a game. could be anywhere from 1%-100%. I would have thought Nintendo would have released Excitebike #'s at some point so not sure why the OP is using reviews as a reference and not just finding out what known excitebike numbers there are.
I don't feel like guessing as to FE numbers. Not a big fan of predicting, in time we'll find out. I hope its a big number and thus leads to more Fire Emblem games in the future. Felt like too long of a wait since Radiant Dawn came out. No I do not count those remakes.