When I did a Slots Filled - Slots, I got a negative number. Seems like it should switch back the operands.

Why do you think this would be a bug? Works as intended as far as I can see.

@rubensworks 81 (slots filled) - 32 (slots) should equal 49 not -49. It's switching the order to 32 - 81, which is wrong.

What inventory are you using? The total number of slots will always be larger than the number of filled slots.

You're right, I was mixing up which value indicated what. Sorry!