New version

Thanks xiota and Carsten S!

I. Define what is the sensor size you are going to use. Let's say you will be using a full-frame camera. (36x24mm)

II. Take a cardboard a bit larger than 10 times that, 46x36cm and make a hole in it, of 36x24cm. We are making a frame 10 times bigger than the sensor. Instead of using mm we are using cm.

III. Put an assistant sitting in the correct place and use the cardboard to plan your framing.

When you like a framing, measure the distance from the cardboard to your eye. Do not poke your eye :o)

IV. The distance in cm (B) from the board to your eye will be the focal length in mm of the lens you need. For example, if the distance is 50cm, the lens you need is 50mm.

Some notes.

If you make the board 10x of the measures of the sensor you will be using you do not need to make any conversion. FC, DX or 2/3 or whatever.

If you do not know yet what camera you will be using you can make a "full-frame" board and then apply a crop factor.

If the camera you are using does not declare the "mm" of the lens or sensor size, but only the field of view, like a webcam, use the "old version" method.

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Old version

Take a cardboard of 40x30 cm and make a hole in it, of 30x20 cm. You can scale this down to half or something and keep the proportions. But is will be more comfortable working with bigger dimensions (and more accurate).

Put an assistant sitting in the correct place and use the cardboard to plan your framing.

When you like a framing, measure the distance from the cardboard to your eye. Do not poke your eye :o)

Now let's use those numbers.

A) Half the longest side of your framing cardboard (interior)

B) Distance to your eye.

C) Put those values on an angle calculator like this one:

https://keisan.casio.com/exec/system/1273849674

Where the base will be B and the height will be A (Altura in Spanish n_n)

You will get the angle C. As this is just half the angle of the real field of view of your cardboard, multiply this for two. (Example below)

D) Take that number and play with this simulator

https://imaging.nikon.com/lineup/lens/simulator/

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Example.

A) My cardboard's interior longest side is 30cm. Half of it is 15cm.

B) The distance to my eye is 30cm

C) The angle on the calculator is 26.5° multiplied by 2 is 53°

D) On the field of view calculator on a DX camera, 53° is around a 28mm lens.

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Keep in mind that the mm and the angle of view is dependant on the sensor size. Some webcam cameras will give you the viewing angle instead of an mm lens equivalent.

Original image: https://pixabay.com/es/photos/chica-joven-estudiante-sentado-3718526/