Take everything you read on any forum with a grain of salt.
15 psi is the "max", but I wouldn't worry if you're seeing 15 psi, your coolant level is correct, and you're not puking.
If you have a sticker on the side of the degas bottle, it sounds like your truck received that update. If you look on the degas bottle itself, you'll see max and min lines indicated on the plastic. The sticker changed (lowered) those levels.
Since you already have the pressure gauge, try this: get your ECT up to 160-180* so the truck isn't totally cold, but you're not at the point of running it hot, either. Accelerate hard. I mean pedal to the metal. Do a couple of quick runs like that and check your pressure in the degas bottle. If you're over 15 psi, you most likely have a problem with your gaskets.
On a side note, Ford began offering an updated degas bottle cap that holds more pressure than the original version. The intent was exactly that: increase the pressure in the cooling system in order to raise the boiling point of the coolant. If you search the site, I'm sure you can easily find the part number and pictures of the new style cap. This cap has solved a number of puking issues, understanding of course that there aren't underlying problems (like a clogged oil cooler, busted EGR cooler, headgaskets, etc).
Any idea what your ECT / EOT temps are when you're running unloaded at 60-65 mph for ~20 min on level ground?