I am aiming at October
Generally speaking, I would say chances of seeing them are very good in November-January in the polar circle as well thanks to the very long nights but snow might not be good if one is close to a city. Usually the sky gets alight because of the reflections from the snow decreasing the visibility of the northern lights. Coastal areas away from cities/towns provide better opportunities as per my experience.
Oh well, then we have the temperature to deal with in that period - not an optimum condition for waiting...
Edit: Kernuak - just saw your website, please disregard
You have been to Tromso I observe. Did you like it?