Hello Helical Insight,
In order to calculate the difference between 2 fields, you can use the minus (-) operator in the typography section of the resultant field.
For example, here we are trying to find difference between Purchase_Order_Hours and Effort_Hours_Billable
Please note that in order for the minus operator to work, both the values should be “Integer” datatype.
In case you have BigDecimal datatype in even one of the fields, you can use the java function in the java.calc library that is supported by the canned reports module.