Ron Bear has given you some great guidance, particularly in showing how to analyze your current time to draw out your likely tempo. He suggests you explore your capabilities in the range between 1.3 and 1.4. That's really sound advice. My recommendation would be to focus more on what SPL results when you swim at
1. Various Tempos between 1.3 and 1.4 and
2. Various Distances between, say, 50 and 200m
View these as a pure data gathering exercise. But it won't only be that.
If, for instance, you swim 100m at a tempo of 1.36 and find you maintain a pretty consistent SPL, you could then experiment to find out what improves your SPL to 22 or even 21.
Doyou save a stroke if you hold Tempo at 1.36, but reduce repeat distance to 75 or 50m
Do you save a stroke if you hold repeat distance at 100m, but reduce tempo to, say 1.40?
Vary both distance and Tempo to learn what combinations allow you to maintain, say, 21 SPL for incrementally longer continuous swims. As you find that easier to do, you could increase tempo by a small bit while keeping distance constant and test your ability to still hold 21. Or increase tempo by a slightly larger delta, whole reducing distance and see if you can still hold 21.
This can be endlessly interesting. I did it for an hour this afternoon at my local 25m outdoor pool, which closed for the season as I completed my practice. You can read my account of that practice here
, as an example of exploring metrics of SPL, Tempo and Repeat Distance.