Inventory Control when Stock not "Sold"

We have a salt therapy business (for horses) (Australia) which uses a non-specific amount of salt with each treatment.
The salt is purchased in 1.5kg bags which will last approx 30 treatments, but there is wastage, usage for priming system and the amount used each treatment is variable depending on the treatment type. The salt is not invoiced.
I have set up the inventory item per bag as a cost of sales expense.
I just want to keep track of bags in stock and order as required.
What would be the best way to manage the used stock? I’m thinking a simple stock adjustment when a bag is emptied. How would I do that?
Does anyone have suggestions?

TIA, Dave

Use an inventory write off since it is a non-revenue adjustment.

Thanks @Tut That’s what I was thinking.