I always err on the side of caution.
As far as B12 is concerned, anything under 600
is suspect, in my book.
B12 taken in the Methyl-cobalamin (not cyano-cobalamin) form, is the best absorbed type of B12 supplement.
I take it sub-lingual (under my tongue) 1000-2000mcg/day.
I've been taking it for over 6 years, and it has helped slow the progression of my PN, significantly.