I do it a fair bit in A2K
The hypothetical situation is: On the sub-form each line has an amount & you want to show the sum of all line amounts on the main form.
1) On your sub-form create a sum control in the sub-form footer, like:
Name:[tab]SubFormTotal
Control Source: = Sum([Amount])
Visible:[tab]False
2) On the main form... create a control to reference the totaling control of the subform... err, that's the one you just created. Sooooooo
Name:[tab]ShowTotalFromSubform
Control Source: =[>>Insert Sub-Form Name<<].Form![SubFormTotal]
As long as you reference the full name of the sub-form's control you can perform any number of calulations on that "Sum()" from the sub-form. & it works for reports too.
i do something like... where i lifted the code
Sub-report:
Name: PAFSubReport
Sub-report's Control:
Name: DocTot
Control Source: =Sum([Amount])
Main report's control: 'shows one month of salary
Control Source: =IIf([PAFLayout]="A",[PAFSubReport].Report![DocTot]/12)
hope this helps