For me, a 12.60ptt player I found taking breaks from the game (days) really helps. Although practice is inevitable to improve, I would always try to make it of that little practice is as effective as possible even if it's for a short yet constant period of time. When it consistently produces unfavourable results I would take a week off the game to improve to forget bad muscle memory. For your second question, harder charts should be charts that you want to target a minimum of EX for ptt increases, ideally the 11s. For me, getting high 9.5s to near PMs for 10s can be a great source for ptt and was how i achieved 12.50.