A couple of things to keep in mind when choosing your foundation. First, there is no one foundation that will match you perfectly under every single type of lighting. The guy at the MAC counter who did my face thought he had found my perfect match in terms of foundation, but when I looked at myself with outdoor lighting, it was a different story. My second point, though, is that there is hope! There are ways to try and avoid that jaw 'line.'
If you really study your jawline under different types of lighting (check out fluorescent lighting, home lighting, outdoor daylight, and outdoor moonlight), you will find that your foundation looks different under each. And this is okay. But try to figure out where you will spend most of your time today. If you spend 40 hours a week in the office, then I would suggest you try to match the foundation to what works under fluorescent lighting, and perhaps keep a different shade of foundation for when you stay at home (personally, I prefer not to wear makeup if I don't have to, plus my hubby hates makeup).
So you're not going to get your foundation to be perfect under every single type of lighting, but you can still come darn close. I would highly recommend that you first take a look at the veins on the insides of your wrists. If your veins are blue, you will try to get a foundation with a cool tone. If your veins are green, you will go for the foundations with the warmer tones. Then go to the store and try rubbing a few foundations on the inside of your wrist. Take a q-tip and remove the makeup through the middle of these spots. I have done so below and written the foundation numbers where I cleaned it with a q-tip.
This way, you can tell which foundation is the closest to your skin tone. Ideally, it should seem like a seamless blend between your skin tone and the foundation. For me, it appears NC30 is the closest to my skin color. It's a tiny bit darker than the skin inside my wrist, but this much allowance is still okay. And try to keep a darker and lighter shade on hand. During the winter you get little exposure to the sun, while in the summer you get a lot of exposure to the sun.
If your forehead is darker than the rest of your face (as mine is), then you can try blending a combination of the darker and lighter shades on your face, with the darker shade on your T-zone and the lighter shade everywhere else. And as always, try to blend really well all over, especially your jawline. Don't be afraid to put foundation on your neck.
These are the only tips and tricks I know, please let me know if you have any others! And good luck finding your ideal foundation!