Madrid is 1.5 hours behind Tehran
Best overlap: 9:30 AM to 6:30 PM in Tehran is 8:00 AM to 5:00 PM in Madrid.
Target 9:30 AM to 6:30 PM in Tehran, which lands at 8:00 AM to 5:00 PM for the Madrid side. That's a roomy 9 hours of common time each day, so finding a slot that suits everyone is rarely a problem. Note that Madrid follows daylight saving while Tehran stays put, so the times above drift by an hour seasonally.