50/50 breakdown

Can anyone give me a short breakdown on the 50/50 rule, or point me towards any books I should read on the subject?

The 50% Rule that I use is just a way of saying that throughout the United States, operating expenses for residential rentals run 45% to 50% of the gross rents. To be conservative, I use 50%. Therefore, 1/2 (50% of the gross rents) go to operating expenses and the other 50% go to the mortgage payment (P & I only) and your profit (cash flow).

Good Luck,