OK, so I'm ashamed to admit that as a native speaker I think I've missed something somewhere. I was typing up some documentation and spellchecker kept bugging me. So I looked up some words and found this:
The suffixes -ance and -ence mean “quality of”or “state of.” Words ending in these suffixes are usually nouns. There is no rule that governs whether a word ends with -ance or -ence.
Even the dictionary on my Mac goes so far as to say:
ORIGIN from French -ence, from Latin -entia, -antia (from present participial stems -ent-, -ant-). Since the 16th cent. many inconsistencies have occurred in the use of -ence and -ance.
Is this for real? There really is no rule?