Intro to BRRRR-- How do I determine my rehab cost?

[Patrick Keene]

I recently watched a video on how to make an offer on a house. It was a simple formula. I think it was:

ARV x 70% - rehab costs - profit

My question is how do I quickly and efficiently determine rehab costs to make an offer?

Obviously, a scope of work from a couple contractors would be the most accurate but it seems a little overkill just to analyze a deal. I could take my property manager or realtor's word for it but that may not be their area of expertise.

Also, I am using David Greene's Long Distance Investing book as a guide so I may never actually see the house. 

Thanks in advance.