I am not clear if you are looking to a Grand Total for the whole report appear or if you just want the field value totals. I will assume you want field value totals based on your comments about the field changing.

 

Two thoughts come to mind, you could add a group based on the field that changes then use a sum. Or you could create a running total and have it reset based on your requirements. The help file is pretty good on walking you through the various control functions in the running total dialog box.

 

Finally, you can mix conditional logic  and suppression formulas to control what you want to appear and when.

 

I am not sure if this helps but perhaps it will give you something to build your solution from.

 

Regards,

 

John W.