Magnitude-to-level conversion is a method of determining how powerful a spell is for AI-altering effects. The equation is thus:
level = magnitude / 4
Therefore, the highest possible magnitude at the spellmaking altar, 100, will give a level of 25. It should be noted that the level is always rounded down. For example, if there is a magnitude of 99, the spell will only affect enemies with of level 24 or less. A level of 1 is the minimum, so if the magnitude is diminished due to decreased spell effectiveness (which happens when wearing base armor instead of shield spells), it will never go beyond one, no matter the magnitude.