I've seen the "round the boat" thing a lot, especially there where students end up doing about 100m at absolute most when you work it out. It's a clear violation of training standards by the instructor. The whole point is they've got a standard metric to work off without guessing - a set distance. This covers them legally amongst other things.
It's bad planning completely to throw a student in for a swim that must be a specific distance without having any idea at all what distance they'll do.
And, at the end of the day, it's the instructor who was there to determine if I met the standard. As described by me, you might believe otherwise, as the guy in the water, and a strong swimmer all my life. I'll give the lean to the guy that's there.