Two numbers with the same name
After repair value is what a property should be worth once the renovation is finished. On a deal you pay for with cash, it is your own estimate and it only has to convince you. On a financed flip there is a second ARV: the one in the appraisal, and that is the number the loan is sized against.
So the useful question is not how to calculate ARV in the abstract. It is how after repair value is determined by the person whose opinion the lender relies on, and how to make your own estimate land near it.