I think that depends on what you are metering from. A reflectance meter (so a simple rangefinder camera meter) reads to create an 18% grey from what you point it at, so if you meter from green grass this reflects an 18% grey and is a datum point. There are other things that reflect 18% grey not withstanding a Kodak Grey Card which is good to use and gain experience from even if you don't use it all the time. So whether you have a meter built into the camera or not you should generally be pointing your camera/meter at the ground somewhere to get an accurate reading, not at the horizon where you can get massive fluctuations in meter readings just by tilting the camera up or down a fraction. So given rangefinder meters, or hand held meters, or accessory shoe meters are not matrix metering just look at the reading when the meter is pointed in the correct direction (downwards) and remember what it says.