I'm sure others have done more extensive testing, but while I'm farming I figure why not log down my success (or lack thereof) in triggering weaknesses. I really, really don't want to level Dancer for its admittedly stellar farming ability here, so I have a particular interest in triggering weakness via magic. I also doubt how robust the bg wiki's percentages are for triggers, or even whether it is complete in scope. For now though, I am assuming most of its information is correct, such as what time of day each monster is weak to magic.
So anyway, here's the results of my first day of recording, it will probably take a dozen or so runs before I'm comfortable with saying, "yeah, that's the percentage".
Day 1
bst/nin, Easy Prey funguar/manticore in Dynamis Valkurm
Full Moon 93% - 95%
212 ninjutsu casts
daytime - Manticores - 8 triggers
evening - Funguar - 4 triggers
midnight - didn't bother switching to sabotender, heh
daytime - Manticore - 6 triggers
Trigger 8.4%
Edit:
Current estimates at average rate to trigger weakness
Magic: 6.4% (208/3261)
Job Ability: 25.6% (428/1673)
Weaponskill: N/A - I dont have a job to test effectively