I use a Peak Critical magnifier and it is spot on. Certain cheaper units were OK, but not as useful for
checking corners of the field. But when comparing respective magnifiers make sure they are all on
lens axis. The better units should match. I've seen cases where budget magnifiers either didn't use
a front-surface mirror or attached it with some kind of double-faced tape that eventually shrank in
thickness. But maybe you've got a bad magnifier or two and should test them with live printing, provided you do rest them on a sheet of the same thickness and apply perfectly calibrated enlgr
settings and a very precise lens at optimum aperture. Forget the blue filter thing and the nonsense
that spacing at the easel doesn't matter. You can choose to shave with a double-bit axe too, but
it might make a difference in your appearance afterwards.