I was considering a Schengen to non-Schengen connection of 45 minutes (otherwise it is a 5hr layover), but decided against it, mainly because if I miss the connection I'm stuck in Helsinki for a night. When deciding, consider your alternatives if you should miss your flight and also if the airline will protect you if this happens.

I was in HEL a few weeks ago and it took about 15 minutes to get through the Schengen to non-Schengen passport control. Not horrible, but with a 45 minute connection if the slightest thing goes wrong, you're not going to make it.