Estimate airport taxi costs.
Airport runs carry costs an ordinary trip does not. Almost every major airport levies an access surcharge on commercial pickups, tolled approach roads are common, and both are passed straight through. Those extras are separated here because they are fixed rather than distance-based, so they weigh most on short airport hops. The tip is calculated on the fare rather than the surcharges, which is conventional — you tip for the driving, not for the airport's access fee.
Airport total
Total = miles x rate + airport surcharge + tolls and fees + tip on the fare
Tip basis
Tip = distance fare x tip percentage, excluding surcharges and tolls
Often, and they remove traffic risk. Many cities set a fixed fare between the airport and the centre; compare it against this metered estimate and take the flat rate if the two are close, since it caps your downside.
Convention is no — tip on the driving portion. This calculation follows that, which is why the tip is a percentage of the distance fare rather than the total.