Question About Applying Analogical Word Building Principles

On the theory that the silliest question is the one I didn't ask:
I was drafting somethng and needed an outline for specifically. What I used at the time was spes + disjoined ke.
When I checked the dictionary later, there was no outline given for specifically, but there were several other -ifically words (Thank you, Andrew, for the online searchable dictionary!). The closest in structure that I found were pacifically and terrifically; one uses joined ke and the other uses disjoined ke.
Is there any general rule of thumb for deciding which of two possible analogous forms is preferred?

(by Denise for everyone)

Labels: ,