One of the four chairs is left off the booking form in every session, and it is for somebody who rings in pain that morning.
Ring reception rather than using the form. They will ask a few questions, tell you what time to come and what it will cost, and if the answer is a hospital rather than this clinic, they will say that instead.
That chair does sometimes sit empty for a whole session. It is still the right way round: a clinic with no give in the diary sends its own patients somewhere else on the day they most need it.