A simple method to gage whether you are likely getting enough UVB from sunlight to make much vitamin D is to check whether your shadow is shorter than your height. Shorter means yes and longer means no. This seems to work both for time of day and for seasonal variations (and latitude variations). In late afternoon or early morning your shadow will be long even in summer. In winter it's always long. And of course if it's very cloudy you have no shadow so again your probably not getting much.