Lastolite makes a 12% grey EZBalance exposure and white balance aid for exactly this circumstance. I've been a long-time fan of the original Lastotolite 18% grey EZBalance for setting exposure and custom in-camera white balance for my outdoor natural light portrait work. But when I switched to the 5D3, also found that spot metering and centering the needle using it still gave me an image that was about 2/3 stop under exposed.
While some categories of images may not complain about pushing exposure in post processing, I find the results in the skin tones when doing this on a portrait to be most unsatisfactory. Seems like the tones underlying the surface of the skin don't scale evenly and so when you start pushing exposure up, you end up with some splotchy looking skin because some parts with brighten up more than others. So for me, nailing exposure in-camera is pretty critical.
I've adapted by increasing exposure in-camera 1/3 to 2/3 stop over what the 18% grey Lastolite meters at. I have the subject hold it right up to their face like a mask. The 12" model mimics the size of the human head/face very nicely so once they have it up to their face, I step or zoom in close, fill the spot meter circle, take my meter reading and adjust +1/3 (or +2/3 depending on the conditions), take a test shot and also use that shot to set my custom in-camera white balance.
The 12% grey EZBalance that Lastolite makes specifically takes in to consideration this propensity of some cameras to underexpose when metering to an 18% grey standard.
My EZBalance is starting to get a little beat up so it's about time to replace it and I'm thinking of giving the 12% grey version a try.http://www.lastolite.com/ezybalance12.php