The other item that must be considered when using Mid-Field customs is your connecting flight's departing gate. If you are in the C terminal, you will be able to make it to your gate in less than 5 mins (The exit to customs/TSA is at gate C7). If your gate is in Terminal D, it is connected to Terminal C and the only way to get there is on foot. There are plenty of flights leaving from D-15 through D-32, so you must account for at least a 10 minute walk (that's for an abled bodied person with minimal bags too). If you are departing to a regional flight, there is a high probability you will be at the A terminal. That requires a 5-7 min walk to the train at C gates, 1 minute train ride, 3-5 min walk to the UAX gates. As long as you are out of customs/TSA by 4:30, you will have no problem reaching your next flight.