My pool swim length is 12.5 yards but the shortest the watch lets me enter is 15 yards. I know can update the lap length from the Connect app after the fact but I usually forget and its after it has been synced to my Strava etc. so I basically stopped bothering - especially since there's no way to bulk edit either. Mentally I just ignore it but would it be such a big deal to let me put the proper length into my watch?
I've heard it said you can't have a shorter length because it messes up the lap detection but at 15 yards setting my FR 255 seems to do just fine - I'd say it's sometimes off (under counting) by 1 or 2 laps per 60 so not bad. I kind of assumed that with the 255 having a compass it would easily detect and average change in heading over a lap of almost any length. I'd be happy to have more battery drain to do more samples with shorter length settings - I'm don't spend huge amounts of time swimming per week.