The thumb tack or tape on the floor really doesn't work well because everyone face is not shaped the same and the complexion and skin tone
light to dark has to be considered. With a formula type set up your hands are tied, you must be able to move your lights in and out to compensate for the different skintones. To work out my set ups, I meter the main light,
from the sitters position with only the main light on. Then I turn it off and meter the fill, figure out the placement of the fill in F stops from what your main light indicated. if main reads f11, the the fill should adjusted (moved in or out) to read F8, F5.6, F4. etc to achieve the ratio you want.
To keep your basic exposure constant: Work out your best ratio and exposure.( Polaroid or test exposures) . When you have found what you like,
tape a length of string to the bottom of your main light reflector. With the model in place and the main set exactly where you determined your best
exposure to be pick up the string and streach it to the models chin or nose.
Tie a large knot in the string big enough that you can find it without looking for it. Next with a ruler mark the string with another knot exactly one foot from the first one (towards the lamp) Next do the same with your fill in it's exact spot. Cut the strings close to the first knot. This is what is called a "poor mans" constant intensity meter. Now unlike the mark on the floor you can move your lights any where you want, but will maintain the same exposure by measuring to the subject with your knots. you may also move the camera position. With a dark complected subject move your main in to the the second knot or perhaps half of the distance between the knots for an 1/2 stop increase. For more or less fill do the same. From that point adjust your background light and hair light. Additional lights should be set in
such a way that they have no effect on your main exposure, but are simply placing a unit or two of light to create modeling or spectral highlight. You may also use this technique with table tops (still life) or product shots.
I actually find it much easier to set up the background light before setting the main, you can see it easier on the ground glass.
You will get 100's of answers to your question, and they will probably all work,
I have taught this method for many years, and it does work very well.