This is not an exact science, and it depends on the area you work in. You have to develop your own set of adjustments and being new in the business I can see where it would be confusing. As ggwwttre above said, find two homes that have sold previously that are alike except for maybe square footage; see how much difference in price per square foot. Then use this figure to figure adjustments in that particular area. These will be different for different price range homes, and different locations. I don't think you can find this in a book. However, a good appraisal class will help. Keep up to date on repair costs by knowing how much it costs for a roof, for example, per foot, or painting per room, etc. It's difficult for a new agent to do this, and it will be time consuming for you.
I take pictures of every room when doing interiors.
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Comments made are my opinion, and not intended to be legal advice of any kind.