Clones/Cloning Help Please.

Im on my 1st grow and about 3 weeks into the Veg stage. I have 16 plants all in the W.W. family w/ an avg height of 15". Theyre still alittle on the short side from me almost stunting them to death but have recovered nicely and are doin great, gowin about 2"/day now. I want 50 - 70 clones from them. Ive done research but Im gettin conflicting answers so I need some help. When I go to cut what should determine when I do? Should it be by the height of the plant?, the # of branches it has?, or which part of the Veg stage its in?, Or a combination? Also, when i cut should I take from the bottom of the plant where theres not alot of light anyway or should I take from the upper part of the plant? Thanx, I appreciate any help.