In a 25y pool, I don't remember all of the numbers, just at the top and bottom laps:
Worked nicely for me, I now have a goal of swimming 200 y at 13 to 14.
After this set, I swam a number of 150 to 200 yard sets at 14 to 15 SPL. Usually, the first 2 laps were easy at 14, and by the time I was finishing lap 3 or 4, a SPL of 15 was hard or impossible to maintain. I would stop the set as soon as I swam a lap where I couldn't hold 15.
Now I am curious about 50 m results for this exercise, I'll try to get to a 50 m pool next week and will post the results.