I'm sorry, but either you're measuring the sag wrong or you're measuring the air pressure wrong. There's no way on earth you'll get 25% sag out of that bike with 300psi in the rear shock unless you weigh the far side of 20 stone. I'm assuming that you don't weigh over 20 stone, of course. If you do, my apologies.
(Air pressure isn't preload, by the way, you're directly affecting the spring rate. The collar on a coil spring _is_ preload but doesn't affect the spring rate, which is a property inherent to the spring -- the only way to change it is to replace the spring).
With rear PROPEDAL off I get just less then a 1cm. The fox sag guide, is probably right for most, I live somewhere a bit flat really, with few bumpy offroad downhills.
Well air pressure (sag), is the equivalent of PRELOAD.
Point being AIR shocks are more adjustable as don't change have to change parts, such as in the case of COIL shocks, change spring, elastomer, etc.
Giant Trance x1 ~ 2008