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?