Appointments + Plugin question

If a service - say Full Foil is 2 1/2 hours long and Tanya works from 10am to 6pm, the calendar only shows available time slots of 10am, 12:30pm, 3:00pm, and 5:30. What happens if someone wants to book at 2pm and there is NO other appts scheduled for that day. How can we allow the user to choose a time and then the system calculates the rest of the day based on what is scheduled? I would love for someone to review how I have this setup and recommend a different option.