--1200 is not all that high a reading; many of us who take large oral doses in the 1000mcg to 5000mcg range daily have much higher readings on blood testing.

As Mrs. D says, you may get a more accurate reading if you stop supplementation a few weeks before a blood draw.

And as Stacy hints, a more accurate way to tell if one is B12 deficient is to have MMA and homocysteine testing prior to supplementation (or after laying off the supplementation for a while).