The way to accurately fill them (or any thing for that matter), is to overfill and then bleed back to the desired pressure. Obviously, hooking up to a gas station air hose could blow out the fork seals in an instant, but hooking up to your home compressor THOROUGH A REGULATOR that is set for 5psi or using a bicycle pump, before bleeding back to 3psi with the gauge that I posted earlier, is easily doable.