I believe one's Vitamin D level is determined by more factors than just the sun. I have mine checked during routine blood work, which gives the bottom line answer. In fact, last Wednesday I was told my D level is a little low.
Since I live in sunny Florida, and do go out in the sun often enough, with golfing, bicycling, outdoor events and so forth, the doctor recommended supplements. I suggest direct measurement in the blood is a better indicator of actual Vitamin D level, which he mentions a bit obliquely toward the end, than indirect methods such as calculating solar intensity. Nevertheless, such a method as provided in the clip might prompt those in low-sunlight conditions to have themselves checked.