In the object I write:

Tsum3 /(Tsum1- Tsum2)

Where Tsum1,Tsum2, Tsum3 are variables defined in the report variables. They have defined wih the SUM function, and reseted by the same data group, and set to NONE calc in the group band...!
These are the values:

Tsum1 = 1922.00
Tsum2 = 51.00
Tsum3 = 2604.60

Correct result = 1.39
Program result(wrong) = 1.31

Thanks
Carlos Burgos