Something I thought would be simple is turning out to be hard for me.  I need to calculate the difference between group summaries.  The simplest way to describe would be to say, that in the image below, I need to take the $4,364.60 and subtract it from the $4,353.82 for each Sales Order Confirmation (GH3) and display that in GF3 (for each Sales Order).  GH3 is the sales order confirmations and GH4 is the sales order date. 

 

I thought I could create some global numbervars and make it work.  You can see these variables calculate correctly when placed in GH4, but when I try to create a new formula that calculates the difference in GH3, it is evaluating incorrectly ({@CurrentTotal} - {@PreviousTotal}).  I also have a reset in GH3.  What am I doing wrong, and is there some better way to handle this?