hmmm the mirror method sounds interesting. First time I've heard of it.
Here is one I've read about but haven't done yet (even though my enlarger is slightly out of alignment)
1) expose a negative on a blank wall, grey card, what ever. and process it
2) scratch a small "x" in each corner of the negative
3) put it in the appropriate negative carrier and in the enlarger (duh)
4) Check focus in each corner and adjust until all are equally in focus.
I don't recall what the aperture recomendations were in the book/article but it seems to be you would want to use as wide an aperture as possible.