How to Make Square Foot Adjustments to Estimate ARV for a Flip?

[DG A.]

Hy BP People, 

I'm working on estimating ARV for my first flip. And, I'm going back through J. Scott's book, and he outlines how to do this. I have 2 appraisals that were done on 2 different duplexes (one I bought, one I did not buy).

I noticed in both the appraisals the only adjustments applied were for:

• Site (Size of the lot)
• Condition
• Gross building area
• Total rooms? https://www.screencast.com/t/eQ5qyYdAuE09
• Garage/Parking
• Fireplaces
• Decks 

I'm having a lot of trouble understanding how appraisers make their adjustments for Gross Living Area / Gross Building Area. There doesn't seem to be a formula anywhere in there. I've been messing around in Excel trying to find a formula or mathematical adjustment to get close to the adjustments the appraisers made (each appraisal from a different appraiser) for Gross Building/Living Area Square Footage.

The whole point of all this is to get a pretty accurate ARV. I'm finding that my number can be off +/- $30,000 depending on how much of an adjustment I make for Goss Building/Living Area.

Here's the spreadsheet I put together: https://docs.google.com/spreadsheets/d/1wT5eKlr81W...

The biggest problem I'm having in the spreadsheet is my calculation for price per square foot of land/lot is way off compared to the appraiser. The same is true for price per square foot of living area. My numbers in both cases are much higher than the appraisers. That said it seems I get pretty close to the "reality" of the ARV in the end.

Questions:

How do appraisers go about making adjustments for Square feet of land/lot and Sq Ft of living area?

Is there a mathematical approach I should be taking that I don't know about?