about cross tab- avarage in summary section

hi, every one.
i have question about cross tab in CR11.

the data in cross tab are following.

____________________data1____data2__...__sum
money amount(sell)____100______200____...__300
money amount(buy)____80_______100____...__180
gross margin__________20______100_____...__120
gross margin rate______20%_____50%____... __?

in normal section, its ok.
however in summary section, i cannot have correct value of "gross margin rate". becouse, in summary section, values are automatically added up(or are calculated based on some fomula).

if you know any solution(or you know its impossible), please tell me...

thanks for reading.what is your expected value there? Try to use formula to calculate that