Rule of thumb for estimating a rehab

Hi all,

Here is a question to you experienced rehabbers. Do you have a rule of thumb ( numbers) that you use to determine how much a rehab is. For example if you go into a house and it needs paint, new floors a kitchen and bath, do you price it out per square foot? or do you price it out on how you will rehab it ( price of the kitchen or bath etc.). 

Thanks for the feedback.

Emma