Yeah, the Iphone will work. You can also go to google maps and get directions from one place to another. If you did not enter your original tire size then program 2400 for the tire size, drive, then do your calculations. Doing this will get your tire size within a few mm.
Another way is to mark the ground and your tire, roll forward 3 rotations, take the length (in meters) and divide by 3. That will give you one average circumference. Then multiply by 1000 to give you mm.
You can't really add a number of mm of circumference for every inch of diameter because different tires have different attributes. Also, tire pressure will have a large influence.
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