This is why I don't price based on a "per plate" rate. IMHO it is FAR easier to do things on a per gig rate. Everyone knows, up front, what the cost and prices are.
If you really need to keep track, I'd guess the plate count could give you a close number, but it will cause an error based on any folks that go thru twice, and/or the inevitable folks that load up plates to take home at the end.
Just my $ .02.
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