Learn to make an amount that is cumulative and display the amount on chart or use to reference from another driver formula
In order to create a cumulative formula you need to first make a New Line in a Custom Table for the Driver to output the value.
Tip: for the aggregation on the New Line be sure to select Balance since the amount is cumulative and you would not want your reports to Sum the amount but rather display the most recent amount
Once you have the New Line you can then create a formula as shown in the screen shot below:
One thing to be aware of is the Start Date of of the formula and to ensure that you have captured the appropriate period.
Effectively what the formula is doing is the formula Driver is referencing itself by the selection of "Output" with a reference to the range of "Last Month" as seen in the screen shot below. You then can add on the amount that you are looking to accumulate in the Rate of the formula.