Predicated on practicable vs practical, is there a more general theme or lesson or motif to be learned?
What are the similarities and differences between two words with the same root X but differ in these suffices, as in: X-cable and X-cal?
This source gives us a pretty small list of candidates that also have an -cal form:
One might add "amicable", as I think I've seen "amical" as a variant form, but there's no headword listing for "amical" in dictionary.com and the words mean the same thing anyway.
So, we have words meaning: 1. "Able to be treated" and 2. "Able to be spoken."
There we have 1. "Related to medicine" and 2. "Related to the voice."
The general pattern holds: -cal means related to x and -cable means able to be xed. However, I really don't think there's a general rule to be drawn from this small a set of examples. I mean, you could memorize a rule about it, but the pattern just doesn't seem linguistically productive so that a rule is more useful than just memorizing six words.