Well I removed the field from the table and put an unbound textbox on the form. I wrote
=IIf([Forms]![StructureT]![BOMF].[Form]![SumOfRequired]<[Forms]![StructureT]![BOMF].[Form]![Stock_Qty],"full","none")
to the control source.
The problem is now, it only checks where the cursor is and apply...