Side boarding is interesting but really only comes into play in best of three games where there is a degree of uncertainty about your opponents deck.
In limited, your side board is the cards you drafted or opened but did not select to be the 22-24 playable in your forty card deck. So in draft, which is how I usually play my cube, you have around 21-23 “side board cards” not all of which are in the colors you are playing. So even fewer cards. Designing a cube, what I don’t want is a handful of cards always taken last and put in the side board. They are situational good sometimes, but usually never played. Ideally every card is desirable but that is hard to achieve if not impossible. For example I’d try to never make a cube slot for a 2/2 for 2 mana creature. It’s filler and unplayable. Anyway, it’s hard to know the dead cards because different people and groups prefer different things. I love counter and bounce spells. But in my 540 card peasant (commons and uncommon only) cube, of the 38 or so blue non-creature spells I want a mix to include combat tricks like cannot be blocked or +1/+1 and flying. So control spells like counter or bounce might be 1/2 or a little more. The reason is to try and ensure some are pulled in every pack but not every spell is counter spells.. Variance a jerk however. It’s tricky.
