In this case, the 35 PSI recommended by the manufacturer (GM) "just so happens to" match the 80 to 85% "rule of thumb" that you have recommended. I say "just so happens to" because I HOPE that GM went through a rigorous test to decide what tire pressure to recommend based upon vehicle handling, design and economy. If they simply stick one generic sticker/recommendation on all of the 2010 Equinoxes, then I am disappointed with the amount of thought that went into the design of the vehicle. I can't speak about what they actually did, only about what I hope they did.
I know that at General Electric, when we design/recommend something on one of our Power Turbines, we do it based upon a rigorous analysis, not merely a rule of thumb. Maybe that is why GE did not face itself with bankruptcy while GM was on the brink.
Thank you for the "rule of thumb" - I will be sure to use it as a last resort when a vehicle that I own does NOT come with any tire pressure recommendation.
As a sidebar : it is Interesting that I just checked my Maxima, and the Max Tire Pressure on the sidewall says 44 PSI and the Nissan recommended pressure sticker on the door says 29 PSI. This winds up being 34% less than the max recommended pressure - outside of your rule of thumb.
Sorry for beating a dead horse... I guess we decided that 35 PSI on the Equinox seems to work, not matter how it is derived !