Correct-- weigh with fuel full, potable water full and whatever you normally drive with in gray and black. Same for people and stuff on board. Basically as you will be going down the road with heaviest normal weight.
The 5 PSI safety margin will not compensate if you start a rock collection or see a 50 degree temperature swing, but easily covers even large Walmart trips.
Don't over-think it, but don't just throw up your hands (like some have suggested) and not address it.
A little overinflated is much better than under inflated, so if you are going to err, err on the side of more PSI.