Of course, this takes all words as equiprobable; results would be different if including the odds of a word appearing in the text into the maths.

I feel like it works more like 90% of the time when it comes up, so maybe this. And could it be that the words where "ie" appears are more ambiguous somehow, like don't fit neatly into some existing pattern?

I don't remember the "after c" bit ever being of use, though, so that part totally makes sense.

Edit: For an example, I'd never forget the spelling of "either", because it's so common and initial letters are more memorable. But, "piece" is tricky - "peice" is my first instinct, and I literally say "i before e" in my head when I write it now.