Maybe % of purchase price (though that would mean low estimates for killer deals, and higher estimates for hot market properties)?

I found this deal off market and found agreeable terms with the seller involving a midterm lease during renovation.

Market rents are above $2k, per door.

My concern here is that depending on where your property is located, that might not be the market standard in that neighborhood.

Hopefully along the way your properties appreciate, you pay down debts  (mortgages) from operating income, and you get market appreciation on what you can rent each unit for - giving you more income each month. 

Made good money for a while then the market turned and he lost a lot of coin ...  

Most buyers don't have the money to come in with to pay down payments, plus commissions in this market, so I don't expect a lot to change.

My worry is that the property might fall into probate or tax auction once he passes and someone who knows someone will get the deal under contract before it reaches public offering.There is a real estate agent in the area that gets about 85% of all the listings in the micro-market, so she is farming these properties constantly. 

I know the market fluctuates and that can be a factor, but generally speaking.

Evaluate the rental market in your area to determine the optimal configuration for maximizing rental income.The choice between using cash flow from your current house to cover expenses or pay off the mortgage faster depends on your long-term financial goals and risk tolerance.