rahul90,
It's rarely a good idea to store a calculated value unless needed for something along the lines of an audit trail, ISO Documentation, or say a one time discounted price that would not normally be provided.
With that said, as Hat2boots has mentioned, the form can be opened in a hidden state. Any value that is calculated on a form either as a local/global or in an unbound control will not be available until the form is loaded. Personally, I would avoid the Access-Macro script language and do this instead via VBA; however, either will work.
Keep in mind, most of us here use VBA for the majority of the non-SQL work unless working with a SharePoint-Application and you will find much more support on the forum for VBA procedures. The Access-Macro language still has significant limitations which is why most of the experts/moderators tend not to use the language unless forced.