For me optimal pacing is sliightly negative split and the goal is to do this while racing as well as in training. So far, on the erg it usually works but on the water is very tricky. However, I've recently become proud owner of a 1x with a Speedcoach, so I'm trying to use some erg pacing techniques on the water (or maybe should do vv).
First account is a very bad example of pacing OTE with an attempt at a 6k 10 days ago. I started with a plan: 2:01 for 3k, then speed up to 2:00 for 1.5 and "sprint for the final 750. Seemed reasonable given some recent 4x2ks and my PB last year of 23:56.
As usual, it started great. Happily rowed a little over 2:00 for about 2k; then still reasonably comfortable at the same speed. And then with 2800m to go I just stopped. I picked it back up and managed to finish the piece in 24:56 eventually but it was miserable. And completely mental of course

The most recent OTW attempt was much better. 2 days ago I rowed 4x1k in my 1x, using my speedcoach Gold as a sort of PM. Some comments:
* The pace varies a lot more in the boat than on the erg so, at least for me it's impossible to be accurate within less than 2 sec. Part of the difficulty as well is that my speedcoach has no average for the piece (don't know if others do).
* On the erg I often divide up the piece into 4 or 5 parts, shifting to a faster pace on each part. Since in the boat I can only discern a shift larger than 2s, I divided up the 1ks into 3 pieces: a 500m and 2 x 250m. The added advantage is that these are often marked points on a course, so that the experience will hopefully translate into situations without Speedcoach.
* In order to start accurately, I now program the speedcoach to start with 10m before the actual piece. This way I "lose" the start, but I'm sure that the piece is recorded.
* When deciding on a plan, wind is critical. I rowed up and down a 2k course twice for these pieces, paddling for the remaining 1k. Head wind is the hardest to plan, because in my experience the differences are larger. For the tailwind "plan" I used my current best times as a guidance and that worked well. Prior to the pieces I rowed both directions with some bursts for warm-up to decide on what pace to use.
For the first time ever I did what I do on the erg for pacing: I chose the pace and let the rate go wherever it needed to be. Quite happy with the result. (quiz question: which are head wind and which tail wind pieces?

1. Plan: 2:10 for 500, 2:08 for 250, 2:06 for 250. Actual 2:07.8 @ 27 (Did not program pieces in individually for this one). Total time: 4:15.6
2. Plan: 2:04 (500), 2:02 (250), 2:00 (250). Actual: 2:03.1 @25.5, 1:59.9 @27, 1:57.6 @29. Total time 4:01.8
3. Plan: 2:09 (500), 2:07 (250), 2:05 (250) Actual: 2:08.2 @27.5, 2:07.1 @28, 2:04.8 @29 Total time 4:13.7
4. Plan 2:03 (500), 2:01 (250), 1:59 (25) Actual: 2:02.2 @26.5, 1:59.9 @28.5, 1:57.0 @31.5. Total time: 4:00.6
Very interested in others' attempts as well...
Veronique