I've never seen a formula but here is a general rule of thumb for multifamily:
The class of an apartment refers to the quality of the building, which can be determined by a number of factors, including age, location, and condition:
Class A: These are the highest quality apartments, typically built within the last 15 years. They are often located in desirable neighborhoods and are close to good schools. These usually have the latest amenities. Pools, gym, etc.
Class B: These apartments are built between 10 and 20 years ago.
Class C: These apartments are typically over 30 years old and are in fair to poor condition. They are often located in lower-income neighborhoods and may need maintenance or renovations. Class C properties are considered the riskiest investment, but they can offer good cash-on-cash returns.
This is a general rule of thumb.
I have a C property based on age and condition that is in an A / B neighborhood. The property when it was built in 1980 was an A property but as time, and ownership changed it has gradually gone down the scale.
If you're talking about single family homes/ duplexes etc. the neighborhood it's in will generally give you the risk profile.