Heal restores health based upon the caster's total wisdom, and also the level at which the spell was cast. If your character casts the spell, that is your current level. If the spell is cast from an item, the level will be the level of the spell on the item as shown in identify.
If your wisdom is less than 100, then heal = 100 + wisdom + floor(level / 2).
If your wisdom is 100 or higher, then heal = 150 + floor(wisdom / 2) + floor(level / 2).
If casting the spell yourself, the percentage practiced is multiplied by the total given above. For example, at 75% practiced the amount healed is 75% of the predicted result.
The wisdom/2 and level/2 terms are always rounded down; e.g. 201 wisdom yields the same benefit as 200 wisdom. The same is true of level 201 and 200.
When cast from an item (e.g. a potion), heal always work at full power, i.e. as if the caster had practiced the spell to 100%.
There is absolutely no benefit in having a level 201 heal potion versus a level 200 potion. The contribution due to level is always floor(level / 2), and it turns out that floor(201 / 2) and floor(200 / 2) are exactly the same.