
and where does chap 85 in the manga start for the novel?

For the webnovel it should be around chapter 80. Also not sure I haven't read the light novel yet but from what I have heard from others is that there are some differences between the two but I don't believe the differences are huge.
Here's a link to the webnovel if you need one: https://novelfull.com/tensei-shitara-slime-datta-ken-wn.html
if he falls for such a cheap trick (╯°Д °)╯╧╧