Pick an interval and either stick with it, make it faster or slower, but leave on an even number that I don’t have to remember along with the time I just did.
I don’t have a dog in this fight 
That said, for that broken 650 set, resting x seconds came in handy: hit the wall, look at watch, go in x seconds. I am not in any performance bracket where the why of it matters, but I liked the change of routine and task of it. That said, all of my sets as a ute were on sendoffs, as you’re describing, so I am used to that sort of workout.
Maybe there’s a coach around who can say why ‘50’s with 10 second rest’ is preferable to ‘50s on :50?’
@PowerPlay you make a good point about Gordian knots in the pool. I actually re-read this thread recently where one of the rules is ‘you shouldn’t have to write it down’ (Joel Filliol, shag, marry or kill). However, I like a good think every once in a while in the pool. There’s something existential about swimming – the silence, the repetition, the grind – that some days I just do not want to get into, and head-scratching gives the workout a little life. Again, I’m not headed to Olympic trials anytime soon, so a little play isn’t hurting my career