While you haven't sent a ton of mail, I would suggest using a bit of "industry standard" numbers combined with your own personal results. As you send out more mail and have more data, you'll be able to refine the number on your own. You sent out 200 pieces, got 12 calls back or a 6% response rate (btw, I'd KILL for a 6% response rate). From those 12 calls you got 1 meeting. Start extrapolating those numbers out:
2000 letters = 120 calls = 10 meetings.
5000 letters = 300 calls = 25 meetings.
Then as you go on more meetings, start figuring out how many meetings you need to go on (on average) to close a deal. Then extrapolate some more. Within 6-12 months, you'll have an extremely good idea of the number of mailers you need to send to hit your metrics.
Finally, there is another aspect here that isn't being touched upon, which is your actual sales/negotiation skills. The better those skills, the better your call to meeting ratio will be and hopefully, your meeting to closed deal ratio will improve as well. So, there isn't one simple answer and there are a whole lot of variables to consider, but it sounds like you are on the right track.