Thank you so much for your post!
I have the same problem (6 page form, can calculate subtotals for each individual page, but cannot calculate grand total of all pages) and have been looking through a lot of content online. Most other solutions involved lots of scripting which I cannot understand or follow or tried to follow, and didn't work for me.
Your solution is so simple and clear to understand and worked great for me.
So again, THANK YOU for sharing your knowledge!!!!