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...Question about gray card: Seems that unless the card is rather large, the image will contain BG that is not 17% gray. So wouldn't that skew the exposure or if you move in closer to fill the image then the flash would be stronger, also throwing the exposure? ...Maybe it would be simpler to ask how far away are you when you test shoot the card, and does the card fill the frame?
Excellent question; you're thinking this through, which is good.
Yes, you need to nearly fill the frame for best results. As I mentioned, I normally use a flash meter in incident mode when setting up for a portrait session, but when I do use a gray card I set my cameras for their smallest metering areas (Spot on the 7D, and Partial on the 20D). That way even if the card doesn't completely fill the frame it will get a good reading.
My old card is 8 X 10 inches, and my newer SpyderCheckr is a little bigger including its hard shell -- both just a bit bigger than the average person's head -- and so when using a portrait lens I can almost fill the frame from the actual camera-to-subject distance; that's required to get an accurate exposure. Due to the short distances compared to the sun, changing the flash-to-subject distance will have a large impact on the exposure levels.
If the flash were on a light stand, triggered by remote or by a long cord, I could move in with the camera as long as the light wasn't moved, and still get an accurate reading as long as I didn't shade the card with my body or the camera.
...I would prefer to have people feel comfortable which is why I am a bit shy about whipping out a 70-200 w hood + RRS WPF bracket/430ex with lightsphere (even though I do use most of this stuff in other applications). It might be easier to plop it all on a tripod and use the remote, and tell them to look at the birdie! I would like to be more comfortable shooting subjects/models with a more direct eye contact style.
If you're comfortable (or at least look comfortable, even if you're faking it) your "clients" will be more comfortable. Practice as much as you can before the event so that you're comfortable with your gear, and be calm but commanding in your approach. Be like Ron Howard directing actors on a set; let them know that you know how to make them look good; show them that they can trust you and your judgement.
From what you've already written, I know you'll do fine.
P.S. -- I almost always use a tripod for formal portraits. I'll stand behind the camera with a cable release in hand, and always look the subject in the eye when I'm ready to shoot. That way, when the subject is looking at me, he/she is looking nearly at the lens. I think that makes the final print more appealing. The "gazing into the distance" look hasn't appealed to me since the '70s, when it was all the rage (and I didn't really like it that much even then).
For a corporate/society roster, it's best if everyone is posed as nearly the same as you can get, and a straight-on head shot with either Butterfly/Paramount lighting or low-ratio Split lighting will flatter most subjects. With one light, Butterfly lighting is the easiest. ...Unless they're wearing hats.
Here are a couple of pictures of my current gray target:
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