I've decided Fate Mastery will have the effect of "all spells cast by you that requires a save have an X% chance to force the enemy to save again in case of a successful save roll.", in addition to the current effects.
We need to decide on the percentage for X next.
You can see how much the success rate of various spells becomes depending on how much it was before in this table :
0% and 100% are obviously unaffected. In general, the effect is the strongest when you had 50% chance to succeed and weakest if the spell had a very low or high chance to work. (This is what makes it very different from simply adding a -1 to the save roll. It alters the outcome most at the spot where it's the most random.)
Even a 100% chance to reroll only increases the success rate from 50% to 75% which is a 25% increase, so it might be our best option?
We need to decide on the percentage for X next.
You can see how much the success rate of various spells becomes depending on how much it was before in this table :
0% and 100% are obviously unaffected. In general, the effect is the strongest when you had 50% chance to succeed and weakest if the spell had a very low or high chance to work. (This is what makes it very different from simply adding a -1 to the save roll. It alters the outcome most at the spot where it's the most random.)
Even a 100% chance to reroll only increases the success rate from 50% to 75% which is a 25% increase, so it might be our best option?