If you stick with the business then you will get to a point where you can give a ballpark estimate of an offer within 5 minutes of the first phone call. I only ask three questions.
1) What's the address?
I always get the address.
2) What kind of shape is the property in?
9 times out of 10 I hear it's the nicest house on the street. I want to hear, "it needs work". I obviously will ask what all it needs if they tell me it needs work.
3) What are you looking to get for it?
9 times out of 10 I get the reply of "you sent me the letter, what's it worth to you?" I do from time to time get someone who will give me a great price, so this is why I ask.
I am rapidly getting as much information from the property appraisers website and the MLS while I am getting the answers to numbers 2 and 3. I am fast enough now that I can keep someone on the phone while I am getting the additional information I need. If I take a long time to get them my offer they might call my competitor because they are so motivated to sell right away.
If my ballpark estimate is acceptable to the seller then I drop everything I am doing and meet with them to view the house and sign the contract.
As far as farming an area, that's for retail real estate agents. You need to cover as much area as you can. Wholesaling is a numbers game; cover as much ground as you can.