Hi @Mitch H.
Someone once told me that before I invested, I should be knowledgeable enough about my target neighborhood to know the going rate vs the list rate of the price per square foot when I see any given home for sale. What will you say when someone calls you up in 4 months from a letter you sent? How will you demonstrate the knowledge that your numbers are more accurate than what they've proposed?
In your quest to get to know the neighborhood, consider starting a spreadsheet that you can save comps that you find on Redfin: list price, sale price, price per sq.ft, bed/bath counts, age of home. Once you get an idea of what things are selling for, you'll be able to have an off-the-cuff conversation with that potential seller and convince them to accept your terms.
While you're at it, I also keep a list of CL rents in the area so that when I see a deal, I generally know what it can rent for. Pretty soon, you'll have all those numbers in your head and will be able to do quick math to analyze a deal.
Keep at the letters and add a little personal touch like @Jean Bolger said and soon enough, someone will remember that nice young man that wanted to buy their home. Good luck!