Write this code in EDIT EXPRESSION of the percentage textbox.
=iif( inscope("matrix1_Prd_grp"),
IIF(ISNOTHING(Fields!ACH_VAL.Value), 0, IIF(Fields!TGT_VAL.Value=0, 0, (Fields!ACH_VAL.Value/Fields!TGT_VAL.Value)*100 )),
IIF(ISNOTHING(sum(Fields!ACH_VAL.Value)), 0, IIF(sum(Fields!TGT_VAL.Value)=0, 0, (sum(Fields!ACH_VAL.Value)/sum(Fields!TGT_VAL.Value))*100 ))
)