In the subreport, create a formula:
whileprintingrecords;
shared numbervar subamt := <your value>;
Place this in the subreport report footer (presumably this is a summary?).
Then in the main report, in a section below the one in which the subreport is located (if your subreport is in GHa, place the formula in GHb), you can add a formula that does a calculation, e.g.:
whileprintingrecords;
shared numbervar subamt;
subamt * {table.amt}/.5
-LB