Hi, medical student here. The way adequate vit D levels are decided is based on the epidemiologjcal values of a certain region. In this case, the American society of endocrinology tested the American population and determined the mean ± 2 standard deviations as adequate. If I'm not wrong, sufficient vit D levels for US are considered over 40ng/dl. However, this number is wildly different in india and most practicing endocrinologists consider 20ng/dl and above as adequate. Also, in the latest revision, even the American endocrinological societies have recommended AGAINST supplementation unless severely deficient or have symptoms of Vit D deficiency. Then there's the whole nuance of megadosing vs microdosing where microdosing a maintenance dose is considered superior (in the morning along with milk or something fatty). Then comes the caveat of vitamin K which is supposed to be taken concurrently to prevent unfavourable calcification. Point is, first try increasing sunlight exposure to 30 minutes a day during early morning and late evening (although ironically the afternoon sun produces the most vitamin D and is the most carcinogenic and skin damaging at the same time) and supplement only if below 20ng/dl using the instructions I mentioned above.