Help me, I'm struggling with the process I want to achieve.
Here is the process for setting the amounts in my Plan B:

I have a field for the Enrolment Fee with an amount of 715, which I will subtract from the Total School Fee of 21,605. The total difference of 20,890 will be divided by 8, resulting in 2,611.25. Let's assume this amount is 2,620. Now, this 2,620 needs to be set for the month of September. 
Like this:

The next step, to set the amount for the month of October, is to subtract the amount of September ( 2,620 ) from the Miscellaneous Fee that has an amount of 3,400. So the total difference is 780. So here's what i want to happen,  I want to set an amount of 2,610 for the months of October to May. Therefore, I need to subtract an amount from the Tuition Fee that has an amount of 16,440 and add it to the 780 to make it 2,610. 
Then repeat the process of subtracting the amount from the Tuition Fee until the amount of 2,610 is set for each month from October to May.

What calculation or script that I need to get this process perfectly? Thanks in advance.

Hey Mark, we have no understanding of your relational structure which makes it impossible to offer solutions.  To start, I suggest you post a screenshot of your graph but far better is to post the file itself.  You can post a clone:  File > Save A Copy As > Empty Clone or create a new small demo file with only the tables & fields necessary to show your issue.

In FileMaker, perspective is everything (well, at least MOST things, LOL).