Milan is 1 hour behind Istanbul
Best overlap: 9:00 AM to 6:00 PM in Istanbul is 8:00 AM to 5:00 PM in Milan.
For the smoothest call, aim inside 9:00 AM to 6:00 PM in Istanbul, i.e. 8:00 AM to 5:00 PM in Milan. The overlap is wide at 9 hours, so you can protect mornings for deep work and still find plenty of meeting time. Daylight saving complicates things slightly: Milan observes it, Istanbul doesn't, so the difference flexes by an hour part of the year.