Not sure if there are any specs for the valve cover. I always tightened the valve cover by feel. In equal steps, diagonally opposite nuts. At some point it gets a bit more difficult to tighten each nut, which is where I leave them be. The most important part is to have clean mating surfaces.
And do get rid off any oil floating in the valve cover before reinstalling it. I forgot that one time, and the oil dripped all over the cleaned head surface when I turned the cover over to re-install it. Had to redo the cleaning. Quite annoying