--is not very high at all, especially for people who are supplementing.

Did you stop taking your B12 several days before the test was done? That might have given a somewhat more accurate reading; if you were still supplementing right up to the day of the blood test, chances are your B12 had been low for a while. Those of us who supplement with 1000-5000mcg of B12 daily often show much higher levels (my last one was in the 1700's); we often have to explain to not-so-savvy doctors why the levels are that high (and why it's OK--we're supplementing, especially as there are no known toxic levels of B12, with studies from Japan and Europe having given up to 36,000mcg daily with few side effects beyond a little hyperness of energy state).

Mr.s D can probably speak to this better than I can--I can say I haven't heard about high levels of B12 causing blood clots, though. I believe that there are some conditions in which both high B12 and blood clots may be correlated, but some underlying factor is resulting in both.