The amount of Hit Points restored through the heal and greater heal spells is based on how many points you have in the Magery skill, at 120 Magery I think it's roughly 54 hit points. On a side note the Spellweaving spell arcane empowerment also increases the effectiveness of healing spells, don't know how much by exactly, you would have to test it, a higher level focus increases the duration of Arcane Empowerment and the bonus to healing spells.