That so-called "normal" scale for the B12 is ridiculous. At 400 and below, B12 deficiency is suspected. But below 300, it is not just suspected, but assumed. Your B12 should really be 500 and above to be in the optimal range.
The best, fastest way to get B12 is via an injection of about 1000. THEN you can try supplements but each person's body is different and there is no way of knowing how much B12 you can absorb, if any, until you do some trial and error.
Based on prior experience, we know that if my daughter gets a shot, it will drop down to 400 after 4 weeks with low B12 supplementation (the amount in most multivitamins). We also know that it will drop down further, to 300, after 8 weeks.
What we don't know is how high do the oral supplements have to be - 500? 1000? So she got a B12 shot three weeks ago, when her B12 was 300, and since then has been trying 1000 oral B12 daily. In another week she will get a blood test to find out whether the oral B12 has managed to keep her from having to get another monthly B12 shot. (But since she won't receive the blood test results for nearly a week, she will get her monthly B12 shot just to play it safe.)