Helsinki is 7 hours ahead of Washington, D.C.
Best overlap: 8:00 AM to 11:00 AM in Washington, D.C. is 3:00 PM to 6:00 PM in Helsinki.
Plan around 8:00 AM to 11:00 AM in Washington, D.C., which is 3:00 PM to 6:00 PM in Helsinki, to catch everyone during office hours. A 3-hour overlap gives you a workable daily window; aim for earlier in it to avoid clipping the end of someone's day.