Terry, it took me a few reads to "get" this practice. Kudos to you for designing and executing it. It inspires me to try a few variations of my own, with stroke counts and stroke selection modified according to my skills.
I really like the idea of doing iso-stroke swim sets at different paces...in your case, swimming 100s at 15 SPL (4 x 15 = 60 strokes) vs. swimming 125s at 12 SPL (12x5=60 strokes)
Also the combo set of 75 + 50 of 2 different strokes...very cool.
How long did it take you to think up this set?