You said, "You stated Building sale: Selling price - selling expenses - cost - depreciation = Net gain...shouldn't depreciation be added (Selling price - selling expenses - cost + depreciation = Net gain)?"
Yes, you are correct. Basis adjustments are added to the cost.
Cost plus sales expenses, minus depreciation gives you the adjusted basis. That is contrasted to the sales price.
As far as land goes, there is no concrete basis for calculating what increased in price over the years and what decreased. It can't hurt to have the same percentage you gave to the land at the purchase be the same percentage at sale unless you have proof that one increased or declined over the years.
