Calculating a text field from data values

How do I assign a value to a text box based off of the sum of a few data values within a database? I've created a DataForm that works well with toggling through each row (record in the database), but I am looking to have a few "totals" fields at the bottom of the form. I am currently just taking the value populated into the text boxes (parsing the string to float values) and then adding them to account for the totals fields. This form is in a tabbed environment though, so the fields that are under a tab do not get put into the calculation when gathering my totals. I would rather get the data directly from the database and then use those. Any help or snips would be great.

Thanks