Sorry for the confusion. I was actually talking about two things. I asked about how much of a downpayment felix made. He bought it in cash so his actual value of N is 100%.
The other thing I was trying to figure out was the theoretical minimum value of N (let’s call it “minN”) that he could have used and still generated a positive cash flow. If minN was 50%, then it probably wasn’t a good deal. If minN was 5%, then he probably got a great deal.