Okay, that makes more sense. Basically, I have two different sets for most of my debuffs, one is a full on potency set, while the other is full skill/macc. For Slow II, I'm casting in 68(/blm)+76 MND, which includes Mythic Wand +1 and Muse Tariqah. If it's something I know I can't land that on, or am having trouble landing it on, I'll switch to the skill/macc set.
But then when I saw the formula on wiki, it suggested that I could go full skill/macc and still have cap out potency. So I guess I was right, too good to be true lol.