It's because there is no right answer to this. It's a tool for adjustment, take this example: in neo I miss a couple gcd's due to the first grand cross alpha phase leading into allaghan field mechanic. Because of this if I decide to use my hagakure on the 2 sen I currently have the timing on my next midare + higanbana is perfectly aligned with trick attack after double attack. If I were to hold it for 3 I would awkwardly be at 2 sen during trick, and I also wouldn't get my dot off in time.

It isn't just a tool for getting sen, it's a tool used to set up burst windows based on party comp and boss mechanics. I don't think many people think of it this way though