This is not a linear gauge. Its built to hang out in the middle through all "normal" operating temps. When its starts to head up past the center at all, its time to check into it.
Do as everyone said and make sure the system is fully cleared of air. If all else fails, put is somewhere with the front higher than the back and *with the system cold and taking great care to avoid the fan blade*, burp the system by squeezing the upper rad hose.