Use the Expression field on the Historic Calculation tab to enter the syntax that defines the Historic Calculation expression.
You can enter the expression syntax in the same way as for other Expression windows (see the ClearSCADA Guide to Expressions).
The browse button provides a context-sensitive menu that allows you to access OPC Tags, OPC Historic Tags or OPC Historic Calculation Tags that can be used to make up the expressions.
If the expression includes a comparison between the data values of two or more points, ensure that all of the source points have samples that occur within their respective sample windows. Also ensure that those samples occur within the overall calculation period (see Specify the Period Over which Historic Data is Calculated). (In effect, the Historic Calculation re-samples all of the historic data that is used in its expression, in order to produce historic data for the Target point.)
For an example of the type of syntax that can be used in the Expression field, see Example Configuration.
For further information on writing expressions, see the ClearSCADA Guide to Expressions.