I always dust the greens for my dragons, and I have always used the RepCal Phosphorus free powder with vitamin D3, it has never steered me wrong. As for how much, I'll take one of two approaches, either I try to achieve a light dusting on all the greens of the top layer, or I will dust the greens and mix it in almost like I'm tossing a salad.
Hope this helps,
Nick Shieler
SnapDragonReptiles