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>Yes, I checked that. The key to this is that on about half the lines it works fine. So if something were checked that shouldn't be or somthing like that, it seems to me that it wouldn't work at all. What's wrong has to be something that is getting changed from line to line. But what? That's what has me stumped.
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Then I only have one suggestion left, and its kinda boring.

Make a listing of the data in the fields affected by the calculation, and step thru and figure each value as you go, boring, but its basic debugging, as I'm sure you already know. I don't think that you have run into any sort of bug, what you are doing is pretty gosh darn standard.

Good luck!