Not sure what type of agreement you may have with this gentleman but some questions that come to mind are: what is your ownership percentage in this project and is that reflected in any contract? 

You will also need a certain percentage of the unpaid balances on your other loans as reserves in order to qualify.  

His percentage he would get off the flips was his income.

If i need to come up with my own money to get started how much and at what percentage.

He has likely been paying a hefty percentage of his monthly payment to interest over the past ten years and is only now starting to see the scales tip in favor or the principal vice interest.

I ask because, even when using market rates, the results I get from each model vary greatly.For instance, when modeling expenses as a percentage of the rental rate, if expenses are assumed to be 50% of rental income, I must have a rental rate of 1.0% of the purchase price to cashflow:However, if I model expenses as a percentage of the purchase price, even if expenses are assumed to be 1.6% of the purchase price, I only need a rental rate of 0.7% of the purchase price to cashflow:I assumed a purchase price of $350,000 in the spreadsheet: at 0.7% this yields $2,450 in monthly rental income, while a rental rate of 1.0% yields $3,500, a huge difference of $1,050 per month.What are your thoughts on the analysis and models?

I am aware of two methods for estimating expenses: using roughly 50% of the monthly rental income, or using roughly 1% (or some other constant percentage) of the value of the property. 

If you purchase a residential income property - like a 2-4 unit - you may be able to apply a percentage of the gross rents to your income, however, some lenders like to see two years of landlord experience to do this - check with your lender. 

Double check to make sure though I could be wrong on the percentages.

An absurdly high percentage of transactions in the DC area are all cash transactions.