There's no such thing as "correct exposure." Especially when one is talking about digital photography.

As Ansel Adams demonstrated with his [*Zone System*][1] almost a century ago, the photographer should base exposure decisions on how much of the scene's dynamic range on wishes to the "squeeze" (for a high dynamic range scene) or "stretch" (for a low dynamic range scene) into the dynamic range capability of the final display medium.

If one obsessively exposes so that an 18% grey card will show up in the exact middle of a histogram generated by the back of camera JPEG preview image, then there's a very real possibility that one has:

 - Pushed details in highlights that one wishes to preserve over the edge and into unrecoverable full saturation.

or 

 - Pushed details in shadows that one wishes to preserve under the edge and into the unrecoverable noise floor.

or

 - Done both at the same time if the scene's dynamic range exceeds the camera's dynamic range capacity.

So rather than blindly setting exposure using an 18% gray card, one should combine the results of spot metering the brightest and darkest details one wishes to preserve in the scene with measuring different exposure settings with an 18% gray card.

In general if one is shooting raw and the dynamic range of the scene does not exceed the camera's dynamic range, one should set exposure so that the brightest parts of the scene for which details are desired should will be just short of full saturation. If the desired result to make an 18% gray card exactly a mid-tone is darker than that, then brightness can be adjusted during raw conversion to bring it down. This will result in a higher quality image with less noise than if one were to expose "correctly" for the 18% gray card and leave the brightest details exposed significantly below the camera's full well capacity.

If the scene contains details that one wishes to preserve that are further apart than the camera's capacity to record in a single exposure, then one will need to create a series of exposures. The brightest exposure will be based on placing the highlights just short of full saturation as described above. The darkest will be based on placing the shadows just above the noise floor. The intermediate exposures will be spaced evenly between the two extremes. The more intermediate exposures one uses, the smoother the transitions from bright to dark will be when one combines the information from all of the exposures using one of several various so called [*High Dynamic Range*][2] image processing techniques.

**Having said all of that:** 

If one desires to expose so that a "standard" (whatever that is, because there really isn't one when talking about converting linear raw luminance values to logarithmic RGB values) gamma conversion curve places an 18% gray card to the middle of of the tonal range of an output image, then one should expose so that The linear raw values are inversely proportional to the color channel multipliers used for white balance adjustment times around 0.18 of full well capacity.

That is, if one is shooting under tungsten lighting and one wishes to wind up with an image that looks "natural" to the eye, so that the colors of objects in the scene appear to be the same colors as when those objects are viewed under natural sunlight, then the linear values of the "red" filtered photosites will need to be higher than the linear values of the "blue" filtered photosites. On the other hand, if one is shooting under very high color temperature lighting, then the "blue" values will need to be higher than the "red" values by inverse proportion to the color channel multipliers that are to be used when demosaicing the linear raw values to arrive at color corrected RGB values.

> So basically when using a digital camera the normalized RGB value before gamma correction should read 0.18/0.18/0.18?

It's not that simple with digital color photography, because you have to take into account what color multipliers will be applied to compensate for the color temperature/white balance/CRI of the illuminating light source when the raw monochrome luminance values are demosaiced and converted to RGB values at a target WB, such as D50 or D65, as well as what gamma correction curves will be applied to each color channel (they can be the same, but are very often different based on CT/WB and HSV adjustments applied during the conversion process).


  [1]: https://photo.stackexchange.com/q/71139/15871
  [2]: https://photo.stackexchange.com/a/80598/15871