personally, i think the mic has a muddy midrange regardless of what distance you are from it. but i use it on vocalists who need the extra "oopmh" the prox effect gives them, and i "make do" with the midrange.
IMO, it DOES seem to have a good amount of prox effect, especially when you're up real close (inside of 4 inches or so). i would start between 4 and 8 inches and work from there. get too far away (a foot or so), and it just sounds *bad*.....thin and cloudy.
don't forget that the angle and direction of the mic also plays an important role in the sound you get from it--be it angled down towards the chest or up towards the nose.....or if the person's singing straight-on or more across it.
there are times when the track calls for one or the other....and also times when it calls for backing up or getting really close....hence my original post--"use whatever distance (and therefore sound) works best with the song you're recording".
cheers,
wade