Take care of few things while passing the value to main report.
- Formula need to be placed on the sub report before passing the value.
- When value is used in main report then the resulting formula should be kept in coming sections in the design.. not in the preceeding sections then you won't get the value.
- one more option would be (
not suggested
)since you are manuplating parameters then link the parameters from mainreport to subreport and use that parameter at any section irrespective of sub report location in report.