Your total fee in any given month depends on the average value of your portfolio over that month. This is why it is not possible to tell you exactly in advance what the final monthly fee will be.
Additionally, in line with your risk class and investment preferences, your portfolio is automatically adjusted and its composition can change over the course of a billing period. Since different funds have different fund charges, and it is not possible to know exactly which funds will be part of your portfolio at a later point in time in the period under consideration, it is also not possible to know exactly what the final fee will be.
However, fund charges are a small portion of your final fee and should not vary wildly from one month to another.