I would recommend doing a little more research on what type of RE you want to invest in, the location or market you're interested in or that supports the strategy you want to use, the asset class you want to or are capable of investing in, etc. After knowing these things you can narrow down the "thumb rules" that you want to use. Just make sure when you decide to use a thumb rule, it's based on the above information. Talk to brokers, bankers, property managers, other RE investors that are in the market you're looking at. Then use the information you find to come up with you're own thumb rules that match what you've found in the real world. Thumb rules are typically only used for very quick, high level analysis and can be dependent on many of the above criteria so you'll get many different answers from different people. Here is a post where the first reply by @Bill Hampton has many of the resources available right here on BP. https://www.biggerpockets.com/...
Good luck!
