Welcome to APUG.

I have a similar grain focuser, except mine is x15 or x20.... (I can't recall) With mine, like yours, if I get too far from center, there is nothing to see. I did see well enough to notice mine was off-alignment because grain wasn't sharp off center. Yes, I do believe, if you can see the grain SHARP, then it is good enough focus AT THAT LOCATION. But since we can't get far enough off center, it's no use.... What really matters is what happens around the four corners.

What's even worse, trying to align enlargers using grain focuser is pretty much impossible. So I ended up (after months of frustrating experiments with less than ideal methods), get a laser alignment tool. It was fairly easy with this tool but even with that, I had to deal with repeatability (or lack there of) of results.