The proper level of vitamin d would be the default level set by our ancestors.Every cell in our bodies have vit d receptors and over 2000 genes are controlled by vit d.It is a critical nutrient.So just like we need X # of heartbeats,X # of breaths and X # of calories from a variety of different sources we need X amount of vit d.The average human living outdoors,when we were in a sunny time of the year or a sunny part of the world made between 5-10,000 iu's a day.This depends on your age,skin type, the amount of skin exposed,the strength of the sun and of course the total time spent in the sun.
You also have to watch the supplements.Independent test showed varying levels of vit d in supplements.The best are the liquid drops.Although studies by a very reputable vit d researcher by the name of Micheal Hollick showed that supplemental vit d only circulated in our bloodstream for 60 % of the time that your own naturally produced via uv light vit d .I would trust my own body to make it instead of supplements.He says that using tanning beds is also an excellent method of producing vit d.Up to 20,000 units from a single visit.