I agree, unless you have a model to work with it is difficult to determine an expectable average of blur.  As a result, I use a system of measurements when creating a scene. 

 

I calculate the size of the object of most interest and then make measurements in relation to that to the distance and size of other objects.  You can do this by taking a length of rod, a cylinder reduced to a size that equals the length of the object of interest and use it as a measuring stick to measure approximately how far away from the camera and other objects; and how large other points of interest should be.  Just switch to one of the other views and measure.

In photography, there are lengths in inches and feet that can correspond to an f-stop.  Trying to achieve scale is difficult when you use aerial perspective, but it is possible.

 

Thanks, I am playing with Vue to see if I might migrate from 2D to 3D, and I think I am hooked.

jankeen.com