Page-Map needed or not? & "Update Manifest Properties"

Hey all...

Finally finished a72k-word, how-to, hobby book. Has 138 files & 120 pics.

Ran through epubckecker and FlightCrew. Good to go.

Used Kindle Previewer (latest) to convert to mobi and the info log, says "Page map not found" (or similar).

I am loathe to try to insert anything into this monster as I fear screwing up something else (since I don't know what I'm doing). Is a page-map necessary? If so, how to I create one and insert it in Sigil?

Last question: what does "update manifest properties" do?

Thanks!