The 50% rule of thumb does not establish equivalency between two investment choices under consideration.
Kirk, in the example you gave at the start of the thread you listed all of the other information one should consider when choosing one investing option over another. All the 50% rule of thumb told you was if you bought EITHER of those and the revenue and expense numbers were in fact accurate, and you paid NO MORE THAN $45,092.27 you are likely to see positive cash flow.
That does not mean either of them is worth that amount; only that the rent and expense structure could carry that amount.
Where people sometimes get tripped up is with SFAs because there are really two entirely separate markets for SFAs. Well, three, but the third one does not lead to this confusion. The two of interest are the retail buyer looking for a place to call home and the investor looking for a house to rent to someone at a profit. The "at a profit" bit is the big clue. For an investor to ensure they will have a positive cash flowing property, the 50% rule of thumb is an important metric to understand.
The other thing to remember is the 50% rule of thumb is useful for analysis, not accounting. At any given period of time, your actual expenses may exceed 50% of revenue, but as a buy and hold landlord, we don't do acquisition analysis in small deltas of time. We are looking at the investment option as an investment that has an acquisition point, a holding time and a liquidation point. When considered over that entirety, the 50% rule of thumb is useful.
The Operating expense study summary report MikeOH referred to is available by clicking here. The study is based on over 33,000 rental units from 83 sources in 26 states. There are links to the more detailed report of the same study and to a discussion on using the 50% rule of thumb with other metrics in the summary report.
If you are a buy and hold landlord and you use the 50% rule of thumb as one of your tools for analysis you will be well served.