The formulas are actually linear, mainly to keep it simple and consistent with the rest of the skills.
This is base damage, but it's going to be more dynamic than similar-level magic. To that end, the weakness multipliers will be less (otherwise, Ifrit will deal 6000 damage at max intelligence, while Fire III will deal approximately 2650). Enemies with normal resistance to a stat will require a further multiplication of four, divided by three, unless the maker lets me use decimals. Enemies reaaaaly weak to something (600 percent weakness) will simply require a multiplication of three, as six would be overkill.
This keeps summons useful for the entire game, but at the same time not overpowered.