Use ArrayFormula on the spreadsheet collecting responses:
1. Insert a column on your spreadsheet (1) that will be used to calculate the grand total
2. Submit your Form once (2). This will create a line on your spreadsheet
3. Insert a row above your Form submission
4. Add your arrayFormula in the row you have inserted, under the column header that will be used to calculate the grand total.
Here we will use=ARRAYFORMULA(C2:C20*D2:D20) or Total=Price*Qty
5.Start collecting form responses. The grand total will be calculated on your spreadsheet, under the column header you have inserted. Reviewers will receive this Total in the Request for approval email
Decline the 1st form submission that was used to create a line on your spreadsheet (cf. point 2. above)
Warning: Form Workflow Plus will detect data on the range of your Arrayformula. This will show on the sidebar as "Not Processed". DO NOT push the "Not Processed" button for this range as this will create FW-ID numbers and send Request for approval emails to reviewers to act on requests.
If you need to process a form submission manually, modify your Arrayformula beforehand to reflect only those rows with Form submissions