Do you use both little (and mana-efficient) spells and big (killer but heavy on mana) spells? You can use double tap to cast them both from the same key. Press once for a small heal, and press twice (like a mouse double click) for a large heal. You can even adjust the response to fine tune double tap to your tap speed.
Tap "A" once to get an "a", tap it twice within 250ms to get "Shift A".
Note if the "Toggle 2nd Tap" is checked the key will be held (like toggle).