ARV rule of thumb?

[Craig Cerny]

Are there any formulas, rules, etc I can use to understand after repair value? E.g. if I spend 20k on improvements what will that do to my ARV? I image if I put 20k into a 500k property it wouldn't have as much impact as if I put 20k into a 100k property? Likewise I'm sure adding bedroom, sqft, etc has more impact than paint and flooring. I'm asking in regards to either SFH or SMF and long term rental, so I'm not considering snow cone makers.