I don't think there's any shortcut to figuring out where a property's income and expenses fall in relation to each other and in comparison to the local market. Without knowing the market rents there's no way to tell where a particular property's potential is. Without knowing what it costs to run and maintain a specific building of a certain construction and age there's no way to figure out if the property is or could make money.
As others have said the 50% rule is just a guide. If a property has high vacancy or rents are very low, the expenses likely are well above 50%... or at least would be if the building was being properly maintained. I mainly use the 50% rule to tell how bovine byproduct free the seller and/or broker are. If a building is well occupied and expenses are shown at 25 or 30%, then Lucy's got some 'splainin' to do.
Whether you crunch the numbers on a yellow pad or a spreadsheet doesn't matter as long is your math is done correctly. That's why I like spreadsheets, because you only have to do the math right once and it works from then on. Whether you build your own or use someone else's is an interesting question.
One one hand it's very hard to get the depth of understanding of how the numbers work by using someone else's spreadsheet but if you're not in the building spreadsheets business there may be better uses of your time, like finding deals. That said I've been building spreadsheets since VisiCalc was new and we have more than 4,000 hours of development and testing time into our deal analysis template. You can see samples of it on our website but 95% of the work we've done won't be visible unless someone's built a lot of spreadsheets. It was worth it to me, our clients and apparently our users.
There are a number of Real Estate MBA courses where the final is to build a detailed analysis spreadsheet from scratch but it's probably more profitable in the long run to hire an MBA than to go get one yourself if you want to be a real estate owner.
