update a bound control on main from subform

Hello I have a Main form with several sub forms on it the tables behind each form are related tables. The user enters data on the main form with the exception of one bound control then moves on to enter data on the sub forms On exiting the record or the main form I want to take data from several of the sub forms and add them together and enter result in the bound form on the main form. Because this bound control is not used to enter data in there are no event triggers to code to get this done any ideas / solutions would be great.



Thanks in advance



Bob