Rehab Calculator?

Hi, I'm new to REI starting out in wholesaling. Any ideas or tips on how to find more information about formulas that could be used to determine rehab costs? i.e., plug in sq ft, location and receive an accurate estimate of a/c repairs?

I'm trying to build a 'deal/no deal' xls and trying to get more specific with detailing out rehab sub categories: i.e., roof, flooring, a/c, paint, etc.. Are there 'rules of thumb' I could build into formulas within the spreadsheet that would give me a good estimate of rehab costs?

Any advice or tips would be greatly appreciated.

Andy