Payback formula - formula to select time period

[bjannseniu]

I am calculating a payback period (savings - expense) on a weekly basis. I am aggregating savings and expenses each week and at some point in time, my aggregate savings > aggregate expenses. The payback period only happens once, when the aggregate savings > aggregate expenses. I would like to choose that week automatically. My example table is below, I'm looking for a formula to populate the Payback Period column as the below example: [TABLE="width: 292"]
<colgroup><col width="56" style="width: 42pt;"> <col width="67" style="width: 50pt; mso-width-source: userset; mso-width-alt: 2858;" span="5"> <tbody>[TR]
[TD="class: xl65, width: 56, bgcolor: transparent"][/TD]
[TD="class: xl66, width: 67, bgcolor: transparent, align: center"]Weekly[/TD]
[TD="class: xl66, width: 67, bgcolor: transparent, align: center"]Weekly[/TD]
[TD="class: xl66, width: 67, bgcolor: transparent, align: center"]Aggregate[/TD]
[TD="class: xl66, width: 67, bgcolor: transparent, align: center"]Aggregate[/TD]
[TD="class: xl68, width: 67, bgcolor: transparent"]Payback [/TD]
[/TR]
[TR]
[TD="class: xl67, bgcolor: transparent"]Week[/TD]
[TD="class: xl67, bgcolor: transparent"]Expense[/TD]
[TD="class: xl67, bgcolor: transparent"]Savings[/TD]
[TD="class: xl67, bgcolor: transparent"]Expense[/TD]
[TD="class: xl67, bgcolor: transparent"]Savings[/TD]
[TD="class: xl67, bgcolor: transparent"]Period[/TD]
[/TR]
[TR]
[TD="class: xl68, bgcolor: transparent"]1[/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,000 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 100 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,000 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 100 [/TD]
[TD="class: xl68, bgcolor: transparent"][/TD]
[/TR]
[TR]
[TD="class: xl68, bgcolor: transparent"]2[/TD]
[TD="class: xl69, bgcolor: transparent"] $ 500 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 300 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,500 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 400 [/TD]
[TD="class: xl68, bgcolor: transparent"][/TD]
[/TR]
[TR]
[TD="class: xl68, bgcolor: transparent"]3[/TD]
[TD="class: xl69, bgcolor: transparent"] $ - [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 600 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,500 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,000 [/TD]
[TD="class: xl68, bgcolor: transparent"][/TD]
[/TR]
[TR]
[TD="class: xl68, bgcolor: transparent"]4[/TD]
[TD="class: xl69, bgcolor: transparent"] $ - [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 800 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,500 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,800 [/TD]
[TD="class: xl68, bgcolor: transparent"]4[/TD]
[/TR]
[TR]
[TD="class: xl68, bgcolor: transparent"]5[/TD]
[TD="class: xl69, bgcolor: transparent"] $ - [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 900 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,500 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 2,700 [/TD]
[TD="class: xl68, bgcolor: transparent"]4[/TD]
[/TR]
[TR]
[TD="class: xl68, bgcolor: transparent"]6[/TD]
[TD="class: xl69, bgcolor: transparent"] $ - [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,000 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 1,500 [/TD]
[TD="class: xl69, bgcolor: transparent"] $ 3,700 [/TD]
[TD="class: xl68, bgcolor: transparent"]4[/TD]
[/TR]
</tbody>[/TABLE]