To compute depreciation equivalent for standard mileage deduction, I multiply business miles each year by the chart, and then do I add totals of all those years together?

Yes.

However, TurboTax can NOT properly calculate the gain/loss from the sale if the business percentage has varied from year to year.  You need to manually report things in the "Sale of Business Property" section (in the vehicle section you would say you converted it to personal use so it doesn't carry over to next year) or go to a tax professional.