I had another go at this yesterday and today. I warmed up yesterday with 4x25 and was pleased that I could do them all in 17 strokes. Then I switched on the Tempo Trainer and started at 1:60. I think my first 25 was 18, after that they tended to be around the 19 mark. I don't think I got back to 17 and definitely didn't reach 16. I went back down to 1:40 or so, by which time I was over 20 for most lengths and then carried on down to 0:90, which was clearly outside my form threshold..
Today I started at 1:10, which I think is also outside my form threshold at the moment and worked up to 1:60 and back down to 1:20. Instead of doing single lengths of 25m I was doing sets of four to get used to the feel of the various settings. I felt most comfortable around the 1:40 and 1:50 settings, so I think for future practices I will try to work outward from this range in both directions.
Again, today, I don't think I did any lengths in less than 18.