Center-of-gravity location, control-surface throws, rate setup, and more good info is typically found in the manual, so I generally rely on it. The recommended settings are great for starting points, but if you’ve ever wondered why some pilots’ planes seem to fly so much better than others—even when employing the same airframe—it could be the mixture of things. The Multiplex 330SC instructions provide some mix values, and when used, they provide pilots with that extra assistance.
Mix #1 Rudder/elevator
So you roll to knife-edge and are thumbing away wondering, “Why am I fighting to keep it here?” To answer the question, first identify what is happening: Is the plane pulling toward the landing gear or canopy? If there is a tendency to pull toward one side or the other, the method of mixing some opposite elevator input to counteract this flight condition will lighten your thumb load. Pulls to canopy?: Create a mix so that, when you input rudder, the elevator will automatically move down, compensating for you. Pulls to gear?: Create the mix to add up-elevator. The percentage you add is key and can best be found through experimentation. Start with a low percentage— maybe 3 to 5%—and adjust as needed.
Mix #2 Rudder/aileron
Your test flights revealed some adverse roll during the knife-edge condition; by mixing some opposite aileron input when rudder is applied, pilots can decrease even more thumb load. Again, test and retest with different values (percentages) and find what works best for you. When you get it nailed down, your knife-edge flight should almost be doable one-handed.