if you are using an invercone or some other device for an incident meter reading go stand out in front of the camera and point the cone at your lens and take a reading. You may want to modify the resulting reading if the distant scene is fairly dark, or fairly light...this is where bracketing helps a lot, by half-stops up and down.
For a reflected reading, point it down at the ground, not at the sky, which is a lot brighter. Green grass is pretty close to an 18 percent gray. The advice on bracketing applies for this method too.