John,
Try this - in your NetWebForm procedure in Clarion, go to the BrowseLineItems (or whatever it is called) field which calls the Browse for the line items on the form.
Go to the 'Client-Side' tab. In the 'Refresh Other Fields' list box, click 'Insert' and choose the 'Nett' field, and ensure 'Value' is ticked. Do the same for the other two fields you want refreshed.
I've done this successfully on normal entry fields, but not tried it on a Browse field. Hopefully when the browse changes, it will trigger the other three fields below to force update.
Let me know how it goes!
Cheers,
Devan