Technical details on spell downranking

[Flymo]

Take the Priest spell Heal 2 as an example:

The spell is learned at level 22, and the base points for healing on the spell keep increasing until level 27. So, level 27 is considered the spell’s max level in our calculation.

Does anyone know if all healing spells increase their healing for 5 levels after you get them, and how much the increase is?