You are correct. When you make a user defined dimension be it an angular dimension or a linear dimension, that dimension is not parametrically tied to the model. Therefore, as the model is made bigger or smaller, the dimension will not stretch or shrink to match the model.

User dimensions are a quick and dirty way to obtain a dimensional value for a piece of geometry that does not have a standard dimension available for it. If you find yourself routinely recreating the same user dimension, then it would probably be of benefit if it were included in the standard list of dimensions.

If there is a specific dimension that you would like added to the list of standard dimensions (which are all parametrically linked to the model), please e-mail me a drawing containing that dimension.