For cadastral surveying (i.e. property / town / state / country boundaries) with corrected GPS readings, we regularly get less than 1cm error per kilometer, which is 0.001% error. Of course, for this you (or your equipment) has to know how far you are from the center of the earth where you are, for which you may have to take the irregularity of the geoid into account.

Even without doing that, the difference between the equatorial radius and polar radius of the earth is only 0.34%, less than the error you impose by not using the exact ratio of kilometers per mile.

It's costs nothing extra to use the exact figure, since it's available, of 1.60934 km / mile. Why use a knotted piece of string to measure when you can use a measuring tape?

Nice calculation website though. Thanks !!