Like most people on here, I could probably talk for hours on the topic, but I wanted to point out two quick things. In all your calculations, make sure you put money away for maintenance and vacancy. I have a couple units that have 0 vacancy if everything is perfect... but even they are going to be empty for a week if I need to put new carpet in.
So, using your first example, I would put away at least 10% for these two things... $150.. so 1 house would be 250 cashflow, and 2 houses would be 300... in my opinion, it's hard to justify another house just for $50/month
Also, My opinion as far as the refinance, which I've done for several properties now: For me, the cash I pull out has to go into an investment... if I pay off cc or vacations, that's just me spending money. If I put the money into another rental... then that's reinvesting. I've seen someone get a HELOC with the intention of buying a rental, then end up with paid off cars, big toys, and their house under water... which is being rented out at a heavy loss since they had to move TRAP. Also, remember to include the cost of the refinanced money in the investment. If your house payment goes up because of the cash-out, you have to add that extra cost to the rental expense... if your mortgage goes up $250/month to buy that one SFR... then there is no cash-flow at all.
Hope this gets decreases instead of increases the confusion... we've all been there.