Stop sum variable from constantly adding initial value

[Ullauri.Byron]

I have an initial balance field that I use to keep track of subtotals and eventually an ending balance.

these subtotals are equal to:         initial_balance + amount 

I created a variable that does this subtotal calculation:         _subtotal

and created another variable that sums on _subtotal:         subtotal

Tbe issue im having is subtotal is adding the initial balance again for each record.

ex.

initial balance:  20

amount = 10         subtotal = 30

amount = 15         subtotal = 65 .       should be 45....

[Ullauri.Byron]

Found the answer here.

1.) Create a variable (_subtotal) with an expression simillar to:  (isFirstRecord) ? initial_balance + amount : amount

2.) Create another variable (subtotal) that sums on _subtotal