overprint black <85% tint

Hello, 

I'm currently using an overprint fix to catch anything in a pdf with black to overprint. Sadly I can only see >85% tints of black and nothing that seems to catch ALL tints of black. 

Is there a setting i'm missing here? 

Illustrator assets haven't been set to overprint and it's an absolute pain to set them to overprint in the files (there's literally 1000's of assets, so it's not an option)

Any help is appreciated. 

Screenshot of fix i'm using

[Edited by Moderator]