If there was a chart or formula, it would be put out by Specialized since the pressure will depend on the leverage ratio and weight distribution on the frame. It doesn't matter where you start as long as it you don't exceed the max pressure of the shock, which is what is specified to be put in prior to pressing the auto sag button. If that's 300 psi, you might start at 250 and see what that yields.
Google setting sag to get a tutorial so you get the process right. IMO, this is superior to using the autosag anyway. Also, some may chime in and say sag isn't important. It's an important starting point. Getting a exact value is not that important. Once you establish a pressure that gives you 25-30 percent, you can vary from there and experiment with spacers to find what you like the best.
I see that Jayem suggests using your weight as the starting point. That isn't a bad suggestion as long as you're using lbs for your weight and PSI for pressure as it just so happens to be not far off for the typical shock piston area and frame leverage ratio. It's just an accident that it's a decent starting point for most bikes.
