Update a calculated field in Gridview via DetailsView

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C#3.5

I have a table with a number of columns containing integer values showing in a gridview. The last of these columns has to calculate its value from the preceding column values each of which is multiplied by certain tariffs held in another table (held there so that they can be updated at any time by uses). What I need to achieve is that the final (calculated) column gets updated when the user updates the other columns from a DetailView. Is this possible? At the moment it will only update when the user calls Update from the DetailsView for a second time - it doesn't get refreshed first time around. I understand why the situation is as it is, but I wonder if there is a way around it so that I can achieve what I mentioned above.

Thanks for any help.

bw

Steve