Ok, l've read this thread and it reminds me of a political debate that has no chance of changing someone's opinion.
From my experience, there are two basic ways to make money owning real estate. Buy and Hold for the objective of positive cash flow. Or, buy and either hold or flip for capital appreciation. To over simplify, I call it the Texas method or the California method. I live in Texas so I subscribe to the buy and hold method for cash flow. It works best FOR ME. Can you flip in Texas for capital appreciation, sure but if your looking to cash flow, it is certainly easier in Texas than California. That being said if your objective is to cash flow then -$100 month is not a positive cash flow situation no matter how you rationalize the investment.
Now to this 50% rule. If it works for you in evaluating property then keep using it. Here in Texas, I'm either applying the rule wrong are it doesn't help me evaluate my deals. I'm afraid I would be leaving good deals on the table by applying that rule.
I went back and applied the rule to properties I have owned from the beginning of 2002. For me the biggest expenses I have for my duplexes (all my properties are multifamily) are Taxes and Insurance. Occasionally the A/C unit needs to be replaced and roof replacement is the other significant expense I have had in Texas. Even the roof is not bad as you usually can wait for a hail storm or strong wind storm to help you out. Let the insurance company help pay for that expense.
Take a typical duplex getting $1000 a side $2000 a month, $24,000 a year. Taxes and Insurance is about $4500 a year and if I spend $3000 a year in maintenance in a year that would normally be high. The 50% rule would say I should be spending $12,000 a year. I come no where close to that. My experience is about 30% expenses a year but that includes taxes (high in Texas) and insurance. I don't pay property management as I do it myself so that might be an issue for some. My vacancy rate is extremely low. In fact the first duplex I bought in the Austin area in 2005 has both renters in each side since I bought it. Also, the rents were $725 each side and now $1000 and I am still $200 below market. I own 17 doors, so I am a small fry in comparison to some of you. But for the last five years I may of had a total of 3 months vacant of ONE UNIT. Now I know many of you won't believe me but the rental market is crazy strong along the HW 35 cooridor around Austin, TX. So think about the 50% rule when my rents were 725 X 2 = 1450/mo $17,400 yearly. So my expenses (50%) would be $8700 and those expenses should now be $12,000 because my rents have increased? So my rents could go up another $200 per side a month if I wanted to raise; that is additional $2400 a year so by raising my rents I am raising my expenses $1200.....really? I guess I could see that logic if increase rents caused additional delays in renting here in Austin, but that just hasn't been the case. Here in Texas I use the 1% to decide if I am even going to take my time to further evaluate the property. The 1% rule keeps me out of trouble and the numbers seem to work here in TEXAS.