Problem with Booking Intervals

I have an issue with the booking intervals. Currently I have the minimum time set to 30 minutes as is default. So when I have a 30 minute service, I can book it on any half hour (8:00, 8:30, 9:00, 9:30, etc). However when I switch to a 90 minute service, it only allows me to book every 90 minutes (8:00, 9:30, 11:00, etc).

This means there are less chances of people booking longer (and more profitable appointments) because there are less slots and if someone booked a 30 minute appointment at 10:00, the 9:30 slot would be gone entirely rather than letting someone else book at 10:30-12:00.

Is there any way to allow anyone to book a longer appointment starting at any :00 or :30 throughout the day?