So the effect is not that bad then. How do I figure out the actual mileage though? If my trip says 10 miles, do I just multiply the 10 by .008 for .08% to get the actual mileage? So the final figure would be 10 x .008 = .08 + 10 = 10.08 miles?
That little difference is definitely nothing to worry about.
Your calculations are correct, but keep in mind that this is only calculating on theoretical dimensions. The actual dimensions are going to be different based on manufacturer specifications, tread depth, inflation pressure, etc. The best you can do is to stay as close to possible to the OEM specs.