I've just finished hauling my P3 full of Mamiya 7 gear (two bodies, four lenses, filters, film, raingear, water, etc.) around Washington State for 12 days. Once I got the fit right it was simply amazing how good it was. I'd ordered it with what I thought to be the correct torso length, but on a first-day trip to the REI mothership in Seattle I discovered that my measurement was off. It took about 2 minutes to change the torso length, after that it was heaven.

Osprey make a backpack fitting/measuring gadget, any good outdoors store carrying that brand should have one.