90 doesn't divide by 20 evenly. 30 is better. No one gets a shorter session that way. 

How about a little over 150, in that case? It is easier to avoid having to use "dimensional magic" that way, like in that Harry Potter tent thing.

The staff can be mostly paid in food and board, and some of them could have grown up there themselves, and thus be used to the way things run already. As for currency, how about copper, silver, and gold pieces? Not the most original on my part, but it makes for easy distinctions between them.