I handle dividends differently and therefore would not be thinking about dividends in the manner the OP suggests. I let them pool and once they've reached sufficient levels to justify making a purchase I see if I have any smart ideas for that money. It may be adding to a position, or starting a new one. I'm trying to buy the most future profits I can for my money while staying sufficiently diversified, which is why I don't automatically reinvest in the same companies the dividends came from.
My other concern with the suggested calculation is that an investor would need to guard against letting it overly influence their decision making. The rate of compounding would be just one factor to consider in projecting total return from an investment. I worry that for a small investor they may weigh this factor too heavily in their mind, in an attempt to overcome their small cash balance. That may result in selecting inferior companies with subpar prospects because of their high yield and low share price. Obviously, some would be better than others at avoiding this potential pit fall.
Minor quibble with how you determined the required investment for Realty Income. Why should they be held to a higher standard than the others, simply because they pay dividends more frequently? I understand not wanting cash sitting in around not invested, but if other companies are allowed to reinvest on a three month schedule, it seems reasonable to me to hold Realty Income to that same standard when evaluating a potential purchase.