Anecodotally, I heard a story about a woman who tested herself regularly. She had to complete a small puzzle she set for herself. Her thought was that when she couldn't do it, she would take action (e.g. move to assisted living or MC). When she couldn't complete it, she also didn't know her plan to move to AL or MC. The problem with the baseline is that you are setting now, and have to remember what it is in the future. A dispassionate 3rd party (e.g. neurologist) can test you regularly, and assess where you are on any given visit.

I suppose you could agree with the doctor on a minimum score before you take some sort of action, and then the doctor can tell you when you've missed that score.

Or you can do as the Count (on TMF previously), and proactively move into a retirement community that offers IL, AL, and MC. He doesn't need it now, but when he does the staff will know and encourage him to the next stage. 1poormom's retirement facility told me when she needed to move to MC because she was needing more attention than they could provide in AL.