I got rather excited when I saw this (being a bit of a spelling nerd), but it doesn't quite work. This website is very useful for searching for words by their spelling: http://www.morewords.com/
and if you put in *ible and *able you get a helpful list of words (though many of them are rather beyond me, and certainly they are beyond 11+).
A couple of examples that don't quite work - corruptible, disputable.
It may be that the rule is a useful guide - I haven't been through to see the balance of it works/it doesn't work.