Ah, my chart also neglects wheel diameter! I think pressure is inversely proportional to the square root of the wheel diameter. The chart is a fit to Berto's graphs for 700C tires. For 26 inch tires, about 10% more should be good.
It is certainly true that charts like this are way too simple. But I would say that the pressure range on the sidewall is also rather simple. Certainly going much over the top is risky. But just what makes a manufacture put a particular pressure range on a sidewall can be rather strange, it seems. There are engineers, marketeers, lawyers, etc. and who wins the debate will vary.
The business of pressure vs rim width is quite fascinating. Just the subject of rim width vs tire width. It seems like wide tires on narrow rims is quite common these days. It's not clear it is anything like optimal. Look at the range of rim widths "approved" for 50 mm tires:
http://www.schwalbetires.com/tech_in...dimensions#rim
Is the best pressure for a 50 mm tire the same on a 17 mm rim as on a 27 mm rim? Yeah my chart totally omits this little detail but I suspect it really isn't so small. It does seem that at least on the narrowest rims you won't want to be putting too high a pressure in the tire because that puts a very large stress on the rims. On the wider rims the angle of the sidewall leaving the rim is not so abrupt so a higher pressure should be less risky.
Definitely on rough roads a bit lower pressure is good, to provide suspension, but not so low as to risk pinch flats. On smooth roads the suspension isn't so valuable and so higher pressure is nice to reduce the rolling resistance. There are a zillion factors. Yeah if you have lots of luggage then suspension is nice but if it's just you on the bike then maybe high pressure is good if you can just hop the bike over any rough spots. Much harder to hop a bike with 60 pounds of groceries in the panniers, especially with platform pedals!
The real purpose of the chart is just to try to open up thinking on the subject. Once you start thinking about optimum pressure as a function of width and load.... it can be a fun space to explore!