350 is not a normal level for B12. This clinician is using, in all likelihood, a very outdated lab reference range.

(I'd even be willing to say 400 would be too low--people have been known to have symptoms of B12 deficiency into the 500 or even 600 range.)

There is no harm in trying daily supplementation with at least 1000mcg of B12--preferably the methylcobalamin form, in case you're one of the minority of people who has genetic difficulties with methylation chemistry (the cobalamin B12 molecule has to be methylated in order to be utilized, and getting methylcobalamin means the body won't have to do this; the vitamin is readily utilizable). By megadosing and getting 1-2% of that dose passively absorbed, you should be getting at least 10-20mcg of B12 daily, hopefully enough to start to rebuild your levels.

You can, of course, take higher doses (many here take 5000mcg/day--B12 has no known toxicity level, although it can be energizing in higher doses).