An oldie, but a goodie. I want to expand a bit on Al Graham answer.
White balance and middle gray are two totally different things and need different tools.
I. White balance
First of all, the white balance is in reality not a white balance, it is a gray balance. Let me explain.
Any color will turn more or less white given sufficient exposition on the sensor. I am pretty sure we all have experience with some overexposed white sky.
So, you can have a real white point on your image (r255g255b255) but the starting color of that zone could have been any color on the original subject.
Now, when you modify a curve, grabbing it from almost the edge make a tiny adjust in a big shift of the curve (red dot). You have more control, and smoother changes if you adjust the curve from the center (cyan dot).
A lot of cheap "gray cards" are not really gray, it is a bit hard to have a really neutral color.
In my experience, whenever possible, the best white balance target is the main light itself. And I am talking mainly on artificial light and specially defused one.
I use "sun" as the starting point for these settings. Close the diaphragm of the camera (f22), set the camera to the lowest ISO (let's say ISO 100) and take a picture of the softbox. Avoid burned zones.
Now use this image as the white balance target. You are telling the camera that this is the light that is supposed to be white.
This method gets a bit more complicated when you have different light sources or when it is a single spot.
Use then a white target, for example, white paper rated with some whiteness level. It can be your seamless background or an inkjet paper. The closer the whiteness number to 100 the better. Use only matt paper. A glossy one can reflect something from around.
Shot this white targets underexposed so they are close to middle gray on your target photo.
Avoid any paper that has some time on the shelf, because paper turns yellow after some time due to oxidation. Avoid paper that has to be on direct sunlight because it can be burned the same way. I prefer a "fresh" white paper than an old "pro gray target".
II. Middle "gray"
This can be a bit more tricky but at the same time, you have many more options to play with.
To have an exposure reading you could:
Use an incident light lightmeter (The ones with a white dome).
There are some domes that can be attached to either a smartphone or even a camera itself to act as a lightmeter.
Use the histogram.
Use the palm of the hand (pretty much inaccurate because there are many skin tones, even in the palm of the hands).
Use some zone system to measure the scene.
Rely on the auto-exposure values, using the shutter priority mode on your camera, and replicate that on a manual mode.
Using a cheap gray card.
Using a white paper as a reference and readjust the values.
I must say that almost any method must be tested and calibrated.
As I am a fan of DIY let me explain how to use a simple white piece of paper as a gray card.
Take a close up of a white paper on the defined light condition using an auto exposure mode. (Use continuous light)
Viewing the histogram you should see a clear zone in the middle of the graph. If the paper was uniformly lit, the graph should be narrow.
- Now, change the settings on the camera to overexpose the exact same shot, let's say 3, 4, 5 steps. If you had, for example, a shutter speed of 1/200s, overexpose using 1/25s. The point is to push this graph to almost the extreme right of the histogram without burning it.
You can do this to the dark side, doing the inverse. Make some tests, use the aperture for fine tunning your tests, for example to 2 2/3 or 3 1/3 exposure changes.
You will now know that if you use a similar simple white paper, you can overexpose an auto mode reading 3 stops more to have an accurate reading for example.
After these tests, you now will know, not only a way to use a simple piece of paper, but you have a method of knowing the dynamic range of your camera. You could find that your camera has better dynamic range and you can push the reading more stops.
But I must say that using a card to measure exposition is sometimes not the best method, because it is strongly dependant on the angle of incidence of the light.
For example, if the light is a frontal one, it will bounce a lot of light, if the light comes from a side it will receive a lot less light, but a 3D subject, like a person's face, will receive a lot of light on that side of the face.
Putting the card on an angle is necessary, probably in a middle angle between the light and the lens.