It's always annoyed me that the tanning tub would have around 40% of the bark left in it, which just happens to not be enough for the next tan. I think one of the following solutions should be applied:
a) We should be able to put half a bark in it. And maybe be able to remove the bark from the tub so you can collect these leftover bark scraps to put in an efficient amount. The bark quantity could be sorta like how a bucket holds 10L of water; bark might "hold" 10 grams of bark in itself, or there could be some container to hold the bark scraps if that would work better.
b) We should be able to put more bark in it without that remainder being wasted, i.e. every 2 tans would allow a third tan with the bark already covered. The tub might be changed to hold 3 bark instead of 2 in that case and it would continue using the same amount it does now (60% of 1 bark).
c) Reduce the bark requirement to 50% so it works for 2 tans.
d) Make the tub use all the bark in one go so people don't question it.