How to find the right dose of vitamin B12?

I have derived a simple formula for vitamin B12 intake calculation, which unfortunately
gives more or less adequate results if the desired level of B12 and the results of the blood test do not differ more than two folds.

For example, to increase from 500 to 900 pg/ml within a month, it recommends taking just 145 ug of vitamin B12 (methyl- and adenosylcobalamin) daily.

exp((target_b12 / tested_b12 - 1) / 0.1607) # ug/d

However, some calculations look very suspicious. Also, the formula does not take into account the factors influencing the adsorption and metabolism of the vitamin, for example, metformin use, age, body weight, etc.

Would be great to run some tests and discuss the results.