The Calculation Field allows you to perform a series of arithmetic computations to a set or a series of fields.
The example below shows on how you can sum up the amount field from a Form Grid.
Figure 1: Configuring and obtaining the field ID from a Form Grid.
In the Calculation Field's configuration, add in a new variable that will map it to the field set in the Form Grid earlier. Match the From Grid's ID and field ID into the variable field ID as shown below.
Figure 2: Configuring the calculation field.