Do a web search for "race sag" and read the Race Tech article and set your sag.
If you can't get the desired sag with the stock springs, then contact Race Tech or Cogent and have them sell you the springs they recommend. Takes all the guess work out of it. As a matter of fact, just contact either of them first and start with the springs they recommend and set your sag.
Just be aware, fixing the front end will only magnify the crappy outdated rear suspension!