Finding the point at which a series of numbers crosses zero, or not

I am doing break-even analysis on a set of data. There are six possible scenarios, and I need a formula that will calculate the crossover point in each of them. In the sheet that I have attached, the six scenarios are given as examples. The formula that I am currently using works in all six scenarios, however, I need a formula that will execute as quickly as possible (hopefully excluding IF() statements and lookups) because the formula will be applied to an extraordinary amount of data...millions of times.

FYI: In the cases where the data never crosses zero, the result does not have to be +/-1000. It just needs to be something that distinguishes those two cases from the rest.

Thanks for any help I can get.

Hi

Please do not create multiple posts on the same request. If you are not getting a response, then reply to the post with "BUMP".

rylo