Cost of repairs; how do you do it?

[Brant Vaught]

I tried to find a thread but didn't see one. My question is for all of you experienced investors. How do you estimate cost of repairs when you go look at a potential fix & flip? Do you have a google doc spreadsheet that you can plug numbers into, or do you have a rough estimation sheet such as house is A B or C rehab, and A is going to cost $X per sq ft to fix correctly? I am assuming you don't have a contractor come do a walk through with you. Or do you just take an insane amount of pictures and go over them with the contractor? If you use a list, is it something that you have gone to the hardware store and priced everything yourself, or would a good contractor know? Lots of questions haha. 

Thank you all in advance!