thank you Kostya....
but I still get the message that this field cannot be summarized.
And your formula 1 is not what I need. As I said the negative amount that is in a group total and not in details should be excluded. But when you do formula1 it includes everything.
Anyway, maybe there is another way to do my calculation.
I need to show the % on each line of group total (group total/GrandTotal)
My details are hidded and what I need on the report is group total and another column with %.