It sounds like your subreport is doing nothing more than populating the shared variable.
Try removing as much whitespace from within your subreport as possible. You can remove vertical whitespace by suppressing subreport sections. I would suppress every section except the section that contains the formula.
Baci in the mainn report, you can try setting the section with the subreport to Underlay Following Sections. This could allow you to still have it run, but not take any additional space up.
Format, Section Expert.
Highlight the section name that the subreport resides in.
Select Underlay Following Sections property.
Hope that helps out.
~Brian