The 18% grey thing is really a bit of a myth, it is a "standard mid-point" of exposure that I think was first mooted by Ansel Adams (wiser heads will put me right on that). In fact camera manufacturers set their exposure meters up to assume the scene they read is 13% grey, because 13% is closer to the real "average" scene reflectivity (whatever average is). If you set a camera based on metering off an 18% grey card then your exposure will be out a whisker for the real situation of the 13% average scene, the difference is not much and will never be seen by print workers, and slide workers should adjust the ISO setting of their cameras until they get the results they like for that particular film, so that adjustment makes things right anyway. The 18% grey card is 18% reflectivity, it's just that cameras assume the scene is 13% reflectivity. Just a bit of technical fluff to clear the air about the sacred 18% grey issue.