Reykjavik is 2 hours behind Helsinki
Best overlap: 10:00 AM to 6:00 PM in Helsinki is 8:00 AM to 4:00 PM in Reykjavik.
For the smoothest call, aim inside 10:00 AM to 6:00 PM in Helsinki, i.e. 8:00 AM to 4:00 PM in Reykjavik. The overlap is wide at 8 hours, so you can protect mornings for deep work and still find plenty of meeting time. Don't be caught out by clock changes — Helsinki observes daylight saving and Reykjavik doesn't, so the offset varies by an hour through the year.