The only way I know of passing a value between sub report and primary report is to use a shared variable.
Declare the variable within the source report, pass the value you want to the variable and then call the variable within the destination report.
For example: You need to pass the value "255" from the sub to the primary then:
shared numbervar var01; (declare the variable and call it var01)
var01:={databasefield}
(assign the value 255 from the database field into var01)
then create a formula field within your destination report, declare the variable and it will be displayed or available for use in a calculation.