This is a trick I use almost every day. If I type something in a document that isn’t in the manuscript, or if I exit Scriv to sync the timeline, I lose my session word count.
Here’s how I deal with that.
I have a document just for setting the session word count. It looks like this:
I select the number of “words” that I want to set the session count to.
I type Ctl-X to cut them.
I reset the session target
I paste the “words” back into the document.