Sometimes I wish Rose were here. She was our specialist on this subject from 1999 onward.

Her thought, which I read many times, was that people vary, and some need more than others. 800 may be enough for many people. But the logic of it all says, if you have symptoms, then maybe you need more than the average. So with time I've sort of come to the conclusion that perhaps the target should be 1000 or more. People with symptoms just are not "normal".

Because B12 is not really toxic, we can be liberal with it. Also it is not expensive either. So if you raise your levels to 2000, ( most tests don't go higher than that--because of the instrumentation limits), and you get BETTER...then that is your answer.
If you don't change at that level, then nothing is lost and you can discontinue. I'd stay high for some time, at least 6 mos to a year, and then you can decide to no longer do it, if there is no improvement.

There are not many supplements that you can do this with. B12 is rather easy to manage. This affords some leeway in making the decision.