Focus on efficient acquisition

You've changed your mind. Rather than grow the donor base, now you want to maximize the efficiency of your marketing spend. Previously you focused on growth balanced with profitability. Now you want to focus on the best ROI because you know each additional piece of mail is only $3 but each new percentile, or batch of postcards, will have a lower success rate.

What is the highest ROI percentile? How much will that cost? What is the profitability of the campaign? How many new donors does the model predict?