Ker-Tables are all well and good, but I don't think it's possible to cast a spell without knowing what spell you're trying to cast. As it is, it's a little restrictive because if you just wanted to cast a simple fireball, you'd have only a 1 in 10 chance. Maybe if you made it less specific, you could apply it to any spell you wanted. For example: on a 1, the spell fails doing xd6 damage to the mage and surrounding units, on a 6, the spell succeeds brilliantly adding 1d6 damage.
Your Ker-Table would be perfect for a minifig who knows nothing about magic trying to cast a random spell out of a spell book just for kicks. In fact, I think I'll use spellbooks in my next battle as objects hidden on the battlefield, and whip out your ker-table when someone tries to use them.