I think Garrison Householder is giving very solid advice but it's also very conservative. People will say expenses at 10 percent.. but that's quite a different number on 3k, rent vs 800 rent. I like to use 150$ per month per door. This has seemed to run true over a few years for me. Some months will be less some will be more.
Capital expenses will occur. Water heater, roof, etc. so I would look at the condition of these big ticket items and estimate how much life you have left out of each of these. That's not to say you won't get surprises, you will, but generally I keep a reserve to handle these things when they occur.
Even if you are cash flow -100. This property may still yells a return if you factor in appreciation, and Texas / depreciation. Also, -100 a month is no big deal if you are getting 1000 a month in debt pay down. People happily throw money at a 401k each month. I would look at in the Same light. If employee(you) pays 100 a month and employer matched it by putting in 1000/mo. Most would take that deal.
With that being said, evaluate your own income and risk tolerance. I wouldn't personally take on too many neg cash flowing deals. But one here and there is not horrible.
Rents go up, debt pay down occurs and appreciation does happen. Better if forced.
40 percent expenses seems to be more realistic. 50 is a very safe number to use though.
