Approximate break even point with LOOKUP

Please view the attachment for more info (tab: klokken-expert)

I have fixed costs (C9)
I have gross margins (C21:C56) that match certain week numbers (A21:A56)

What I would like to know is in what week we break even. So basically it should add up the margin for each week (ignore #N/A) and return the week number when the SUM of the previous weeks equals or > than 0.