The Patersons rely on your focusing on an aerial image. That's why you adjust for the line to be sharp.
The grain isn't on a surface but is supposed to be seen at the same plane as the line.
I've never been able to use one myself, I can't seem to wrap my mind(eyes) around how it works. I end up using the image on the base board & reading glasses. A large diameter magnifying glass might also work.
Your link tells me that it is the Paterson Major Grain Focus Finder. I have both Minor and the Major and your are right about how to focus. It is the line across the middle that needs to be sharp. Get this sharp either with glasses or without then depending on whether you got the line sharp with or without glasses use it with or without glasses.
I have used both of mine with and without glasses and they seem to work OK with or without glasses. I am puzzled as to why you cannot get the grain into focus. If the grain is in focus then the neg will be also in focus except in the situation mentioned below.
Be aware that if the neg itself is out of focus then the grain will still come into focus but the print will be out of focus and never will be in focus. Nothing will make a fuzzy out-of-focus negative sharp.
"Be aware that if the [image on the] negative is out of focus then..."