Ansel Adams always recommended 18% gray, but ANSI seems to differ, and some cameras' built-in exposure meetings seem to be offset to the left.
With modern cameras it is less and less useful. Their metering is very advanced and use big number of points weighted according to algorithms.
If you are using spot metering you are usually will become familiar with camera for quite short time.
Doesn't matter as long as you know where to put each one. They only exists as an objective standard to be placed where they would fall under ideal conditions.
So use either. Putting 18% at 50IRE is just a tent pole. A way to know that in that particular lighting condition you are exposing correctly via that pivot point.
But your DSLR's meter is calibrated for 12% which is why it will always be off compared to a light meter. So it sits at the bottom of zone 5, where as your light meter will put you in the middle of zone 5.
In-cam meter - bottom of zone 5 12% gray<
light meter - middle of zone 5 18% gray<
So use an 18% card and either accept it's +/-0 reading at like 42IRE, or adjust your exposure by a 1/3 stop to move it up to 50IRE.
do with this as you see fit.
Problem is that in real modern cameras default metering is much smarter. As most of cameras now even use special condition databases where they select proper metering methods (like beach, snow, etc), some use face detection to provide most weight to this image areast in metering. And it'll be more and more advanced.
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