Updating

I want to make an update code without referring to the form

My update query reads in is sql as follows
Dim SQL As String
StrSQL = "UPDATE products SET products.new = [products].[grossprice]-[products].[grossprice]*28/100"
I want to rewrite it in the following way :
Dim Diff As String
Diff = [products].[grossprice] - [products].[grossprice]
SQL = "UPDATE products SET products.new = diff*28/100 "

However this effort fails since i have to refer to a form.Can i build a code without referring to a form?