I've found a Genova Diagnostics NutrEval to be most useful in customizing a program.
Thorne MethylGuard is a good product, as are MethylGuard Plus, Neurochondria, and 5-MTHF. There are similar high quality products from Seeking Health and Designs for Health. 5-MTHF is the form that your body uses, you don't have to convert from the other forms, so it's easier for most people's bodies to use.
If you start down the path of folate and B12 supplementation, be aware that you may also need more magnesium, molybdenum, B6, B2, B1, amino acids, and possibly TMG, potassium, and methionine. How much you need of each will depends on your genetic and environmental factors. A functional medicine doctor could be extremely helpful, but you can learn a lot these days on the internet (though the quality of info varies).
As for dosage, you'll have to work out a plan customized to you, rather than following some recipe for someone else. It's best to start with a balanced approach and gradually increase/adjust doses.
Some people can tolerate only the tiniest amounts, while others of us need massive amounts. For example, I recently learned that though I'm taking 250mg B2 (riboflavin) a day, I'm still severely deficient, even with 19,000% of the "daily value" for normal people, which is not a good idea as its essential to making ATP, our body's energy currency. (It's also one of the deficiencies noted in the metabolomics study, so not too surprising.)
Best wishes...