How do you calculate the fee when someone leaves or joins in the middle of a month?
The rate adjusts every time a slack user is removed or added. There is a pro-rata mechanism. So for example, if you spend 1/3 of the month at 180 users and 2/3 of the month at 120, the rate will be: (1/3 * 180 + 2/3*120) * ($_Current_price)