How do you determine ARV?

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I'm curious how y'all determine ARV (After Repair Value) in your area. I'm looking at some foreclosure's and the hardest part seems to be what they would be worth after repairs. I have a great estimating software to give me the actual cost to rehab, but have no idea what it could or should be sold for once finished.

Do you have your realtor pull comps and base it of a price per sf? If so, do you use an average, lowest, etc???
I've been using comps then base it off the price per square foot they sold for. I usually figure conservative for worst case scenarios.

Another question, is there any place or software or site to determine what a property will appraise for? I'm wanting to see the appraised value "as is" before any repairs.... Id like to be able to do this before I buy the property. Of course I would hire an appraiser once I made my choice and get a professional decision....