shared variables...
in the main report before the subreport is called a formula like this will reset the variable:
shared numbervar visits:=0;
""//will hide the zero on the report

in the subreport, probably the group or the report footer:
shared numbervar visit := count({table.field});
""//again to hide the total visits

back in the main report...after the subreport
shared numbervar visit;
referal count / visit //or whatever the true logic is

HTH