Strap in, this is gonna be long and possibly confusing.
I have to test and record the first and last data point in groups of 25. Each quarter, the iterations change, so in Q1, I'm testing 1, 25, 26, 50, 51, 75, 76, 100. . . In Q2, I'm testing 2, 24, 27, 49, 52, 74, 77, 99. . . and so on...