think that's all very tight to the other EVF-thread. Reminds me a bit on a podcast from Mat "I'm not the same swimmer every day"... Most of us will know it, sometimes there are days when everything will fall into its place and we think, that's how Terry and Shinji might feel... and, sad enough, we don't find the grip to reproduce the same next day. And sometimes there is a day where we're not sure if anything is in the right place, and everything seems to go banana included (what should never happen! so we should better take a time out in the whirl pool...) the joy of swimming some laps. No, I won't suggest the whirl pool. Better will be being curious and swimming some "bad" laps to feel what's happening and finding some fun in all that "banana-stuff".
Preserving our joy of swimming for me is most important over all other things. And so I'm sure there are so many things that can not be put into numbers. RPE is a try but it's subjective and not the same every day.
In OW I can swim longer, much more relaxed and further than I only can think in a pool. That will brake down if no support-boats there or am too far from the beach... (Am a coward with that...)
Sometimes I swim my 1000s the same time as the other day by time and feeling RPE, but I swam with two SPL more or less, not being able to tell what changed.
Most times I swim my last two laps (LCM) with today least possible, slow and overgliding strokes; differs from 28 to 32, can't tell what happens today for that number... but swimming that way is not as effortless as swimming with 38 to 42 SPL...
When going for an (nearly) "all-out" the last 50-100m SR increases and SPL decreases (down to 4SPL). Strange, but that is my feel to get my momentary fastest.
Sometimes I focus in special things. FE kick-timing or more "aggressiv" breathing or more forceful spear, what shares 1-3SPL, but can't integrate it into my cruising-pace with the same result.
Most times when I ask my wife to have a look (videos are strongly forbidden), when working on a special FP. She says: No difference... but I do feel extreme differences.
And so many of our decisions where to work for improvement might be more a matter of feeling than a matter of pure calculation, but calculating may be a helpful tool sometimes, as goggles or a TT. Sad enough not having Terry's genius to find always the right things to improve in the right way. (Damned, missing him!)