400 IU is the RDA. On the other hand, the RDA assumes 20% will come from food and the other 80% from sunlight. That is, 2000 IU/day is actually what's expected

400 IU is the RDA. On the other hand, the RDA assumes 20% will come from food and the other 80% from sunlight. That is, 2000 IU/day is actually what's expected from all sources. Enough to get 25(OH)D in range is the more generalizable rule. Such an approach as you mention may work well enough if growth is finished (ie, plates fused) and there's a movement away from calcium to magnesium. That is, get more magnesium than phosphorus and then 1/3rd-1/2 as much calcium. For example, 1.5g phosphorus + 1.5g magnesium + 500-750 mg calcium. As much calcium isn't needed after growth and I am assuming all RDAs are being met (with just a variety of whole quality foods before adding supplements). Especially enough potassium to ensure a negative PRAL score to further lessen stress reactions. At such a low amount, you could try the sun/S.A.D. or red lamp therapy.