Damn, I thought I checked the posts, if someone had asked it. Now I see the other post there too.You are doing it correctly, as I mentioned above when someone else asked the same question.. you shouldn't cancel a cast just to use Firestarter proc.
Also, waiting for your Fire to hit just to see if you get a Firestarter proc is a huge dps lose over time if you do it for the whole encounter. I would seriously advise not doing that.
Anyway, any idea which secondary stats are best (like everything except intelligence)? So far I have been treating them pretty much equal. Not so much variation in gear either to be able to choose things except with materia. There was some discussion earlier about INT, but that is a mainstat. I'd think that faster casting speed would be most beneficial, but I don't even know how and what the other stats exactly do so everything said about this is just a feeling.