Istanbul is 2 hours behind Karachi
Best overlap: 10:00 AM to 6:00 PM in Karachi is 8:00 AM to 4:00 PM in Istanbul.
Shoot for 10:00 AM to 6:00 PM in Karachi; your Istanbul colleagues will see that as 8:00 AM to 4:00 PM. That's a roomy 8 hours of common time each day, so finding a slot that suits everyone is rarely a problem.