@theducks - you have me slightly confused now
. The new option has no user interactivity with it whatsoever, you set it once and forget it.
By default being turned on users who are happy with how the plugin works today will see zero difference. If however you turn it off, then what happens is the plugin looks to see whether you already have a value in the page/word count column before deciding whether to even to kick off the background job to get a count. So if you already had a page count, and selected the estimate page count option, it will straight away tell you it isn't going to do anything for that particular book. End of story, no "option" to do it anyway etc. If you decide you want to do want to overwrite a value in one specific case, then rather than going back into options and ticking the checkbox again you could just set the value to zero.
How does that fit with what you had in mind? I'm trying to avoid having more prompts - people already complain about the (unavoidable) one that comes up once the plugin has the counts and is ready to update the library...