Appointment system

The appointment calendar time slot size cannot be changed so that it is different for each practitioner. I have two doing 45 minute slots but want to add one to have 20 minute slots. The problem with that is the person booking has to be careful to input the correct start time to ensure no gaps between patients. Also, they are unable to view a blank diary page and easily see which appointment slots are available for each practitioner. It would be so much easier, and surely its doable, to have alloted time slots? This is a basic requirement for a booking system. I’m sure I can’t be the first person to query this.