Class of neighborhood, population growth or decline locally, job growth or decline locally, amount of new construction, percentage if home ownership but having watched rents since the late 80s on my parents houses and having my own properties for a decade or so I haven't ever seen significant declines in rent I have seen rent stagnation and rent growth but never any real drops out side of a couple areas where neighborhoods declined greatly.

@William C.Partner is getting his percentage of the deal because of his credit.

Main question is what is the lowest percentage you will go for rental properties before you consider buying. 

I also find that 60% is closer to actual value of expenses in my local areas - after you get a few properties under your belt, you can find what percentage works for you.3.

If you want an FHA or conventional owner-occupier loan, you'll need more than $10k to cover the minimal 3.5-5% deposit for your next $350k primary.While in theory, 2x Equity Lines Of Credit could be approved, remember that Lenders only give out a percentage of the homes' values, so, you may not have $74k of borrowable equity!

I would calculate a percentage from existing listings using sales data and the appraiser's website.

Lot of risk to take to accommodate a fraction of a percentage of current users. there are digital methods that are cheap, fast, easy and require little infrastructure already (ex: venmo)

As a percentage, it's hard to tell but you're still looking at several thousand dollars, depending on the property tax rate

Other than that you're numbers look pretty solid, and the expenses you calculated as a percentage of the income seem to be pretty reasonable.

I used the bigger pockets calculator but can't get it to calculate the Cash on cash percentage.