scheduling appointments of different lengths consecutively

I would like clients to be able to use my online scheduling to schedule appointments of different lengths, with the same provider, consecutively. For instance if someone schedules a a 15 minute brow wax appointment when we open at 10 am, the next person, who wants a massage as early as possible that day, should be able to schedule a 90 minute massage at 10:15. However, on my schedule page, the 90 minute massage shows up as available at 10, 11:30, 1:00 etc. So when the 10-10:15 appointment is scheduled, the next possible time available for a massage is at 11:30. This means my practitioner is sitting around doing nothing from 10:15 -11:30. Your help is greatly appreciated - thank you in advance!