assign calulated field value in grid

Hi
I know you can ass ign a value to a calculated field in the oncalcfields
procedure, but how can this be done in response to a double click on a
dbgrid ?  I have tried putting the following in the doubleclick proc.

dmQuote.AdsQItem2Orderselected.Value := 'Y';
 but it does nothing.

Regards

Andy DYble