you sweet spot it the same as with any autococker based gun. Mine was done from the factory. I would have to check but I believe mine is flush or really close to flush. If you dont know how to sweetspot an autococker then look up the halfsquat41 channel on youtube. There is a sweetspot video there.
the velocity adjustment is through the back like on most autococker based guns. Unlike other pumps/cocker, the j12 uses a 1/8" allen wrench and not a 3/16" allen wrench. The spring on the j12 is behind the ivg but the ivg is reversed threaded so it is still clockwise = + and counterclockwise = -