That would be fair. After all, we don't know how many pages of formulae are needed for each spell. I know in DnD, the higher level the spell, the more pages it took to scribe in one's spellbook. Which would make sense - channeling more aether for a larger or more powerful effect would likely require additional computation and calculation.
So that could be a balancing factor. One could tattoo the effects of a small spell without taking up too much surface area, but something like a perpetual Stoneskin or Protect field would require quite a bit more. I would figure making something perpetual instead of cast and done would require additional calculations as well to make it self-sustaining anyway, would would add to the length of the formula (and thus, the size of the tattoo).
So that could be a balancing factor. One could tattoo the effects of a small spell without taking up too much surface area, but something like a perpetual Stoneskin or Protect field would require quite a bit more. I would figure making something perpetual instead of cast and done would require additional calculations as well to make it self-sustaining anyway, would would add to the length of the formula (and thus, the size of the tattoo).
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