master archivist

[Mudbill]

I believe only notes are required for it, not diaries. Which means you should get the achievement when you get the last note in the Chancel Redux map, just before the Inner Sanctum.

Anyway you can check the last save game you have, count how many notes you have, then compare it to what I collected in this video:
https://www.youtube.com/watch?v=RGhOXX7N69c