Here are the correct descriptions :
Additional effect 15 : modifies the target's Strength/Magic/Defence/Magic Defence by [M] - 100%
Additional effect 16 : modifies the user's Def% (=physical evade) by [M] - 100%
Additional effect 17 : modifies the user's Strength by [M] -100%
Additional effect 1C : modifies the target's Defence and Magic Defence by [M] -100%
So, if [M] = 200, the above stats will double (+100%, the maximum). And if [M] = 75, you get -25. So the stats will be reduced by 25%, etc...
Also, there are two things to note with Additional effect 16... First, I'm really not sure it works on enemies. I tried to give Rufus a high Def% that would decrease everytime he'd laugh, but I never felt he was easier to hit, even after his 10th laugh. Not only that, but just like in RedXIII's Lunatic High, giving this effect to a multi-party spell will further increase the caster's Def% as more allies are hit by the spell (Lunatic High's [M] is 150, but it will likely double Nanaki's Def%, if at least two allies are alive).