Calculations on an Access 2013 form

Dear All,

I've been having a hard time doing calculations on an Access 2013 form. 

I have a database of subscribers who pay their subscription on a monthly basis. Each subscriber is given a unique MemerID. I have two tables which are in a one-many table relationship, i.e. each MemberID has several subscriptions which he has to pay per month throughout the year.

I am trying to design a parent-child form with details of Members as parent and their subscriptions as a subform.  Each member makes a pledge for some money to be paid each month. Suppose a member pledges INR 200 rupees (Indian currency) every month. Then the total amount pledged annually would be 200 x 12 = 2400 rupees.

I want to calculate the amount due for each MemberID each month. Therefore, I used a Totals Query in an Access 2013 query and grouped by MemberID. I summed the amount paid each month for each member by doing a Totals Query to get the Total Subscription and then I intended to subtract this total amount given with the Total Amount pledged. This should give me  the Amount Due for each MemberID for each month of the subscription paid.  The subscription amount given each month should be a cumulative total or a running total which is then subtracted from the Total Amount Pledged which would give the Amount Due for each MemberID. 

But, I'm having a hard time designing the query and using that on the form. What is the best way to calculate the Amount Due for each MemberID on a row by row basis each month till the Total Pledged Amount becomes 0 (zero) at which point, the member would have given all his Pledged Amount for that year?

A screenshot is attached to give you an idea of what I have designed.

Can anyone please help me?

Kind Regards,

~Maneesh