Actually, the chance for a cash deal to cash flow is almost always 100%. If you don't have a mortgage payment and even if your expenses add up to 50% of gross rent, you'll still have amazing cash flow. The real metric you need to evaluate is the cash on cash return vs your ROI.
Assuming 100% down payment (e.g. all cash purchase), you would divide your annual net income by your purchase price and the resulting % would be your ROI.
Assuming financing with a down payment, you take your annual net cash flow and divide that by your down payment and your result is your Cash on Cash Return.
I like to go with a hybrid of both. You have to assign a cost to your money even if it's only for internal metrics. Whether or not you actually have to borrow money, all money costs something. If you're using it for an investment, the alternative is to lend it out to get a return. Let's assume we're all capable of lending at 8%. You have to assign your down payment an interest expense at 8% and subtract that from your cash flow before you get your CoCR.
Good luck!