the part nobody prices in
A chair-hour is inventory that expires every hour.
An airline seat and a two o'clock manicure slot have the same economics: when the moment passes, it cannot be sold later. You do not carry today's empty chair forward into tomorrow. It is simply gone, and the technician was paid to stand there.
Which is why "don't miss calls" is the wrong frame for a salon, even though every vendor sells it. Missing a call at 5pm when you are fully booked costs you a future appointment — annoying, recoverable. A 1:50pm cancellation that nobody backfills costs you the two o'clock itself, and there is no version of tomorrow where you get it back.
The two problems need opposite things. Not missing calls is inbound, and automation is genuinely good at it. Backfilling a gap is outbound — someone has to work the waitlist, ring the client who wanted Saturday, offer the slot, and handle the "can you do it for a bit less" that comes back. That is a person, and it is the reason the US$650 shared tier is the one we lead with for salons.
Calculate break-even using the contribution from each recovered appointment after variable costs, rather than the full ticket price. Divide your monthly support cost by that contribution and round up to a whole appointment. Count only appointments that would otherwise have remained unfilled. If an appointment makes no positive contribution, recovering more of that work will not cover the support cost.