To expand upon my previous post, all bound queries have a data source (where the information is coming from). It is either a Table or a Query.
If it is a Query, all you need to do is add a Calculated field to your Query that does the calculation that you want (so the calculation is done in the Query and not on the Form)
Code:
MyCalc: DSum("MONTH0_Active","qry_OnTimeDelivery_MetricData")
Then, just pick that field from the Field Selector on your Form to add it.

If your Form is using a Table as your Data Source, create a new Query that adds all the fields from the Table that you want, and then add the Calculated Field, like above.
Then, change your Form to use thre Query as the Data Source (instead of the Table), and add your Field from the Field Selector to the Form.