What if you're not full to capacity? And since the charge for the first hour is different than the charges for additional hours, and then there's "sequential" and "concurrent" charges, whatever the heck those are, then you could never be consistent throughout a day. Your method assumes that all chairs are full, that someone is in each of those chairs every hour of the day, AND that there are only initial-hour charges.
And that is never going to be the case. If you'd stop worrying about accounting, and just imagine this happening for real, I think you will realize that's never going to work. At 2:00 in the afternoon you've got 5 chairs full, one person has been there 3 hours, another just got there, another has been there a half hour; two hours ago all 6 chairs were filled; one person is almost done and then 2 chairs are going to be empty. Now don't you think that's a little more realistic?
When I said not to do it by the customer, I meant that you don't have a "per customer" charge that you can multiply out by the number of customers. When you say to calculate by "the number of customers," that's meaning there's ONE charge that is the same for every customer so that you can based on it how many customers you had. But that isn't the case.
But that doesn't mean you can't figure out what a charge is for any one individual customer. But that charge is figured based on the hourly charges.
You're kinda making this difficult, not keeping it simple. If you walked into this place as a customer and stayed 2 hours, you'd have the initial first-hour charge plus an additional hour. So how much are they going to charge you? They have to figure that out anyway to know what to charge the customer, right? Any time you have customers, you have to know what to charge them. Again, I certainly was never saying you can't figure out the total charge for any given customer. You have to.
Then add up the total customer charges.