Neville -
I measured a couple of courses with a JO counter mounted on the bike but using my BC600 4-magnet cyclocomputer for all the layout. When I stopped to take data it was Jones counts I recorded.
Then I acquired a Protege 9 and rigged it for one-magnet operation. I now use it in place of the JO counter. I use the BC600 as a resettable device for split layout and only record data from the Protege, which keeps an unbroken string of data.
I do not like measuring tricks that are unrecorded in the data, so I never permit the Protege to roll past 9999. I generally stop at, say, mile 10, take a count, and reset to zero again at that point. My string of data at this point might look like:
Mile 10 - 7547.23
Mile 10 - 0
I have measured a couple of courses electronically and have high confidence in the method.
Whether the Protege skips a count at 9999 or 10,000 may have been misobserved by me. Perhaps it was 9999 as you suggest. I will keep an eye open.
I have noticed that most of the people I've corresponded with who use the electronic method are using spoke counts. I believe they would profit from marking their rims in 20 equal segments, every 0.05 revolution. It is a boon to be able to get a direct decimal reading I've found. Marking the rim can easily be done in a half hour, and once done, it's done forever. Well worth the time.