lamb61, your GI is giving good advice, as levels should be at least 400. In fact, anything below 300 is presumed to be deficient even though it is in the "normal" range.
Too many GIs are misled by the lab report into thinking anything in the "normal" range is OK. It isn't, because "normal" does NOT mean "healthy."
In fact, presumably, since the normal range falls below 300, then it must be normal for much of the population to be B12 deficient.