Optimizing this formula?

I have three columns and only calculating col 2. I have a heading so my
calculation starts from row 2. But here is the task.

1) row 2 is my opening balance.
2) every row after this is a amount that is subtracted from the row 2.
3) the last row will contain the closing balance.

I'm creating a monthly budget and this is what I'm able to do so far.
A) {=B2-(SUM(ABOVE))} BUT, I can't seem to add an extra field to subtract
the extra balance.
B) I used {=B2-SUM(B3:B10)} BUT, I don't want to keep changing the last row
number.

Any suggesting on how to optimize this or am I stuck with making the last
row number change?

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Coding is like a box of chocolates!...