The US lab ranges go down to 200... and that is no longer considered safe. Significant neuro symptoms occur for people below 400.

I recommend the PNers stay over 1000.

There is no upper limit for toxicity for B12 assigned by research or the FDA. Some of us maintain over 2000.

The higher the blood level, the higher the level of B12 can get in the spinal fluid. The gradient between the blood and CSF is driven by concentration of the serum. So the higher the serum level, the more that crosses into the spinal cord and brain.
There have been some studies on low spinal fluid and MS and Fibromyalgia. So keeping good blood levels of a 1000 or more is
a good idea.
B12 in supplement form is best taken as methylcobalamin which is the active form. Orally one should take on an empty stomach since only about 1% may be absorbed. People with intact intrinsic factor may absorb a bit more, but most of a high dose of 1000-5000mcg is not absorbed passively. Presence of food will negate most absorption, since the food acts as a sponge and prevents the tiny microgram amounts from reaching the intestinal wall during transit.