The determination of what is the best light ratio to use in portrait photography is subjective. Portrait photographers like to use different lighting ratios for different effects.
A lighting ratio is the ratio between the main light (also known as the key light) and the fill light.
Traditional portraits, like this portrait on the left, usually have a lighting ratio somewhere between 2:1 and 4:1.
A 4:1 ratio indicates that there is four times (or two f stops as much light striking the highlight areas of the face than is striking the shadow areas.
Having such a range will still produce good details in the shadow areas and provide enough contrast to reveal your subject's form.
The photography lighting equipment used does affect how you adjust the distances to change the ratio. Small light sources follow the inverse square rule for light fall off increase with distance increase.
Using a light meter is the most accurate way to take measurements, but who does that anymore?
This diagram shows a typical lighting set up that is popular today. Umbrella lighting is a very affordable way to create attractive portraits.
Instead of using another light, a large reflector can be placed close to your subject. Placing it closer to your subject will give you a lower lighting ratio.
Placing your reflector at a farther distance from your subject will mean that less light will bounce back into the shadow areas. This will produce a higher lighting ratio and a more dramatic portrait.
With the advent of digital photography, it becomes much easier to "cheat" and simple look at your digital camera's LCD screen to preview the lighting effect you are achieving.
Because digital camera sensor's are getting so sensitive and capable of shooting in low light situations (high ISO number), it is becoming more popular to shoot without flash.
I use natural light from windows when I am shooting portraits inside whenever possible. No fancy lighting equipment is needed.
The curtains diffused the window light to create this beautiful, softly lit portrait. The curtains also happen to hide any distracting objects from outside the window.
I used a large reflector on the right to bounce the window light back on my subject. This is one way to get this relatively low light ratio.
Another advantage to using this photography technique is that you don't have to wait for a flash to recycle. The light is continuous.
You can make many exposures in a short time frame and you won't miss any great portrait expressions. You just have to pay attention to shutter speed and make sure it is not too slow so that you avoid any motion blur.
Try a few different light ratios by changing the distance of your reflector from the subject and then compare the results.