Rules of thumb are OK for eliminating multiple prospects, but at some point you have to look at trailing financials and use them to make your future assumptions.
After you have looked at a few deals you will begin to get an average of certain costs, not their $ amount but their ratio of gross operating income. From there you can refine your prospecting by using some of those assumptions on properties you don't have the full picture on.
For example, as a baseline I use 8-15% of all gross income as my fixed expenses, 10-15% for capital improvements/reserves, 5-10% for vacancy and 1-5% for loss to market. If you have a property management fee include your 3.5-10% off the gross. These numbers obviously don't include debt service. I also feel like I'm forgetting something, but you get the idea.
Then I use 2-3% for inflation across the board unless the market dictates otherwise.
These are still "rules of thumb" and if anything can be used as guides or as a quick replacement for a lack of information. Always input real numbers when you have them, and then ask yourself "How good of a manager is the seller? Can you improve on those numbers? Or if this is your first time or two is it possible you make a few mistakes? If so add some cushion in your numbers, use 15% instead of 10% for variable expenses.