Here's a fun fact about how the game works, for helping optimize player preparation. Note Prisha's form on the left is 15.9, but only 13.9 on the right 'detail panel'. This is because the summary on the right updates everything at the start of the day. That's when the match result is calculated, etc. But on the right, it shows the 'in real time' form.
So in this case, I actually have a performance penalty because my form isn't high enough. I've traditionally thought that the number on the left was the 'accurate' one, and so I would be up at 15.9, and not get hit with that penalty.
In terms of applying this, it's always best then to never be below 15, even at the very start of a week. Otherwhise there's an experience penalty like I have here. Also, if you're using experience to train and want it to affect a particular match, you need to do that
before the day changes to the day of the match. On the match day, it's too late. It's also important to remember this idea when trying to set up a top player for the optimal form zone (20-25) in a big tournament.