Consistent Appointments lower limit behavior for varying durations

Hi.
After a client missed an appointment because one of their clients booked a slot later than was thought possible I am led to believe that the appointments lower limit setting is offset from the appointment end time rather than start time...is this correct? If it is correct, is there a way to mimic a situation in which it is triggered from the start time instead? My client has 30, 60 and 90 minute bookings and it is odd to tell them that whatever value they set for the appointments lower limit will have a different effect depending on which duration is booked: for example, if they set this to 2 hours then 30 minute bookings will have to be booked 1.5 hours in advance while 1.5 hour bookings can be booked only 30 minutes in advance.