Realistic travel time from Tarangire to Kilimanjaro airport

As the title states . I’m considering booking an 18.40 international flight the day our safari ends. Our last day is spent at Tarangire. The night before we spend at Karatu. Would we need to cut our last day short? Tour operator said we’d most likely be back at Arusha around 4pm so she thinks it’s doable. Thanks in advance.