Thank you. I was thinking that a 100% spell was immune to any deduction. Thank you to those who posted the explanation nicely.
A quick clarification -- which only matters under certain circumstances like battling Luska in Water Works -- but to make sure there is no misunderstanding:
Like blades, accuracy boosts and debuffs work as multipliers and dividers, not adders and subtractors.
For example, casting a 100% spell with no native boost to accuracy. Add +20% Accurate TC. Add +20% Infallible Aura TC. Add +15% Guidance TC. Add -50% Ink from Luska.
The answer is NOT 100+20+20+15-50=105%. If you think this way, you will be surprised when you do occasionally fizzle. I certainly was. I even filed a bug report. KI politely thanked me for submitting the report. :-) It was the first time I ever encountered not getting a real response from them on a bug submission.
In retrospect, they must have been laughing all the way to the "Closed -- behaving as expected" button.
The answer IS found thus:
100 + 20% (.20x100) = 120 120 + 20% (.20x120) = 144 144 + 15% (.15x144) = 165.6 Rounds DOWN to 165 165 - 50% (.50x165) = 82.5 Rounds DOWN to 82.
You have an 18% chance to Fizzle in the above circumstance.
And even the above math is not deadly accurate, as someone has done a fine analysis and shown that W101 doesn't use decimal mathematics when it comes to buffs and debuffs, but rather a bitwise approximation. It is possible to be off a few points if the numbers get big enough.
But I digress.
Like blades, accuracy boosts and debuffs work as multiplers, not adders. Each step in the calculation adds or subtracts a percentage of the running total.
It just so happens that with 100% and -50%, you get the same answer both ways. :-)