part heading number continuation?

I’ve got 2 boxsets, books 1 - 4 and books 5 - 8. I’ve created a part heading at the start of each of the books, and in the title options I have this…

Book <$t:part>
<$img:<$custom:Image>;ebook=60%><$rst_chapter>

That works perfectly for the first boxset and shows ‘Book one’ etc plus the cover image below.

But on the second boxset which has books 5 - 8, it’s obviously going to display the book xx incorrectly. I’ve tried something like…

Book <$t+5:part>
<$img:<$custom:Image>;ebook=60%><$rst_chapter>

And variations on that, but it doesn’t work.

Is there a way to add five to the number so it works correctly for the 2nd boxset?