Star...I get the math, and understand that it is indeed for the most part fair. I did something similar when I was a BM at a very large boarding facility....we had 45 horses, and had certain horses on a certain schedule...so I too get what you mean and how you manage it so it works out for all the different needs of the horses....
That being said...I didnt like doing it that way, as it was alot of work to keep everything straight.
Now w my own facility all I do is charge the boarder the fee on that months bill.....So if the farrier comes say on May 10th for instance... they will get billed that month for work done. So they can pretty much expect a farrier fee every 4-6 weeks depending on the horse in question.
I pay my farrier up front...if he comes trims 5 horse he hands me the bill, I give him a check...done. No chasing money, no chasing boarders....as I know they will pay me when they pay that months board.
Works great for the farrier, works for the boarders and works for me. Easy peasy...
Now if they want to use a farrier other then mine...they have to arrange it, and pay him themselves.
Horses are like chips...you can never just have one