10 - 15% is acceptable - I would tip 1/2 on the each leg of the trip - as you may not get the same driver. You will pay for both legs of your RT when you take the first part of it.
Hmmm....10-15% of the total (roundtrip) or the equivalent of one way?
We usually give the driver $10 each way. And I agree it's best to tip 'each way' as there's a possibility you may not have the same driver coming from and going to the airport.