When I shoot a model portfolio, if the client does not have a beauty shot, that's where I start the session. Because there's no wardrobe involved and hair is pulled back tight in a pony tail, it takes very little pre-shoot time to get this shot in the can. Makeup is typically benign, mostly eye makeup and some lipstick so again you're shooting within a relatively short period of time after the session begins. The idea for the shot is to emphasize the model's natural beauty without a lot of makeup, accessories, or clothing to compete with the face.
Some photographers set up this shot with only one light, a beauty dish centered, above, and pointed down at the model's face at a 45 degree angle. I use this set up as well but I find that using just this one light creates a noticeable shadow under the model's chin. I have experimented with reflectors, foam core sheets, etc. under the chin but the amount of bounced fill light is just not enough for my taste. My solution is to add a second light with a small soft box in front of and below the model, angled up at her chin at a 45 degree angle. If the beauty dish is metered at f11, I'll experiment by metering the second light at f5.6, sometimes pushing it up to f8 and even f11, depending on the look I'm after.
Here's the final image with the setup depicted in the previous image. I was going for a very high key kind of look which is easy to accomplish with the lighting setup used. Here are a few more shots from the same session.
For a brighter, super high key effect, try a large soft box instead of a white seamless backdrop and meter it at f11. My 4'x8' soft box is perfect for this and when used it adds a soft envelope of light that wraps around the model's face. Here's an example.
So, whether you go with the super high key look or something a little softer, try using that second light under the chin to eliminate the shadow created by the key light above the model's head.