I don't think BobK's rule was Latin-dependent.
BobK?
Oh dear, I seem to have acquired an eponymous rule ;-) And to make it worse, I've found numerous counter-examples. Some of them are obscure enough for most people to ignore them - abbatial, equinoctial, inertial and interstitial. But others are not so easy to explain away: initial, nuptial, partial, spatial... there are probably more. Thanks to fivejedjon for the
-nc- exception - all the ones I'd found were -
[no N]t- .
To answer the 'Latin-dependent' question, my surmise was not
exclusively Latin-dependent. I was saying that in order to predict the spelling it would be necessary to know a word's provenance. Latin comes into the story in many cases.
Back at 'the rule'. I think it works as a rule of thumb. So far we've only come up with a handful of exceptions, and there are very many more 'regular' examples (Latin
regula [='a rule'];-) - all the words derived (either directly or indirectly) from a Latin present participle. for example: substantial, penitential, existential, influential ... etc etc.
b