Reykjavik is 1 hour behind Frankfurt
Best overlap: 9:00 AM to 6:00 PM in Frankfurt is 8:00 AM to 5:00 PM in Reykjavik.
Target 9:00 AM to 6:00 PM in Frankfurt, which lands at 8:00 AM to 5:00 PM for the Reykjavik side. The overlap is wide at 9 hours, so you can protect mornings for deep work and still find plenty of meeting time. Note that Frankfurt follows daylight saving while Reykjavik stays put, so the times above drift by an hour seasonally.