I think this is a valid and interesting question, and I hope we get some useful answers here.. but I personally think the market structure is also important, i.e how saturated it is and what not...
also if you know how big your market is I believe to some extend you can think of how much of a market share you can get, and with those two you can obviously estimate how much your company would be worth (on a good day).
If your question is to know how much money you can raise, the estimated value of the company will obviously help you a lot. If the market is worth $100M and you will only have a percentage of that market, I doubt you would be able to raise millions of dollars from the VC's, and if you do, you are probably looking to give up a lot of shares.
But if there is a "rule of thumb" I would like to hear it too