it may not behave exactly as per formula but hell everything in structural world doesnt exactly behave per formula.. aisc gives you an estimate that works..
I would size my shear lug based on shear capacity only, thickness of web would all that matters I suppose? So shall I treat my shear lug exactly as a cantilevered beam?
My experience is that shear lugs are designed as cantilever beams. I have also seen this done in different parts of the world as well as in NA. Most people use AISC Design Guide 7 Industrial Buildings or something similar as guidleine.
I can't think of another, at least simplified way, of performing the design other the cantilevered beam method. Simply looking at the entire shear force acting over the width (not depth) x the thickness does not seem right to me because the force is distributed. In a short lug the shear may govern over flexure but I have never seen that to be the case.
