The first rule to see the northern light is (obviously) a clear sky. There tend to be more days with clear sky the father you get from the coast.

Not sure what your schedule is, but if you are into a small adventure you could try Ivalo in Northern Finland. Domestic flights from Helsinki and a few guest houses/motels to stay at. If you are in Norway anyway, Pasvik in North-Eastern Norway is easily accessible through Kirkenes (daily flights via Tromsų to Oslo). This is the western-most border of the Russian tundra and a truly amazing place to experience the arctic winter.