I'd hope for stacks instead of chance to apply.
Say that you can stack the buff, and there is a maximum multiplier. Certain monsters (like bosses) will be more resistant and will require more stacks to hit full effect - for transparency once you hit max stack it could tell you and then save it in your bestiary under monster resistances (Boss bla bla 10 stack resistance, meaning you wont get full potency till 10 stacks - some may be immune like those already undead, and you may get more complex like 10 stacks to max stack but even then you aren't at full capacity so it could read "Undead 12 * 75%" and you would know at 12 stacks you will get a max of 75% efficiency rather then what you'd expect)
As far as I know there will be a bestiary in FFXIV :P
Example for those going wtf is that:
Also could turn the buff name once at maximum to "turned undead" or something since you completed the process on them (though those who you cant get a full efficiency on would never turn all the way).