Base time and padding of appointments

have set up Appointment+ on this site: Time base is 15 minutes;

A customer can pick and appointment time on the quarter hour, however if I add a 15minute padding to the service provider after a service this changes to only give 30 min slots

I want the customer to be able to choose a start time of on the hour 15 past 30 past or 45.

the appointments are either 1 hour or 90 mins, however can start on any quarter hour. so for example I can have one appointment 9:15 to 10:15 then the next 10:30 to 11:30