Need help Nailing the arv correct

[Alphonso Link]

Hello, Biggerpockets family. How do you find the appraisal adjustment numbers. When calculating arv and comparing property I would like to have educated estimate on what the adjustments are on a extra bedroom or a garage versus no garage and etc. Can somebody please help me with this information it's been days I been trying to figure this out. I want my arv to be correct..