Winston's turnaround and ride back technique begs questions. The arithmetic is a little easier by using a simple number to recalculate the remaining split and finish (or start) point locations. However, this divides the course into segments that have to be detailed on the certification request. I have not heard of a measurement that had 2 segments for one ride and one segment for the second ride. Would certifiers accept this? I don't see why not as long as the explanation was complete and accurate.
Riding back to the last known, marked location then manually spinning the wheel (with attached Jones Counter) backward to the same number as when the point was marked and recorded is arguably less confusing and requires no explanation of segments.
Obviously, starting over from the beginning also works and may be easier for a short course.
The easiest method we have used for backing up a significant distance (more than is practical for manually spinning the wheel) is to turn the bike handlebars (and wheel) 180 degrees and ride carefully back to the last known point. A little care is needed as the brakes are on the wrong side of both the handlebar and the hands (squeeze with thumbs instead of fingers). In addition, steering takes some care when the front wheel is backward. This has proved to be surprisingly accurate in that a manual adjustment of only a few counts is required to get the counter back to the last marked and recorded number. Of course bike cable length can be a prohibitive factor.