Trouble getting total of fields in subform.

Trouble getting total of fields in subform.

Hey! I've got a form that has a subform called "project worksheet". That subform has a row with 2 fields (length and width) that get multiplied together, and then divided by a number, the answer (sheet usage) is then displayed on that row also. I need to get a total of every "sheet usage" field, displayed on the main form. What I have is mostly right, however the answer that is displayed on the main form appears to be the sheet usage from the first line of the subform, multiplied by the number of rows in the subform, NOT a total of those fields. Can anyone help me out with the script for this? Thanks!