Originally Posted by ro63rt
I must say when I was in the pool the other morning my easier lengths had a stroke count of 20.
That's telling you something. And yes, it's about maintaining momentum. It sounds as if you may achieve 17SPL by introducing small pauses in your rhythm.
Get adapted to 20SPL--able to swim steady reasonably good (for your current level) paces and gradually add distance.
Then to 19 SPL, etc. Eventually you'll reach 17SPL and feel the same. Then you can use the full range of Green Zone counts.