If you’re going to make a viewing card, you’d want to design it so that it works for a viewing distance that makes sense for your arm length, if you want to hold it at arm’s length, or somewhat less. In my case that would be about 500 mm.
For a viewing distance d and angle of view θ, the correct opening dimension w is
w = 2d*tan(θ/2)
A 90 mm lens used on the 4” x 5” format (95 mm x 120 mm) gives viewing angles of 55.6º across the 95 mm dimension and 67.4º across the 120 mm dimension.
The minor dimension of the opening in the card is w1 = 2* 500 mm*tan(55.6º/2) = 527 mm
The major dimension is w2 = 2*500 mm*tan(67.4º/2) = 666.9 mm
That’s a large, cumbersome card.
It could be scaled smaller for a closer viewing distance to reduce its size to something manageable. Using a scaling factor of 1/3, we’d have an opening of 175.6 mm x 222 mm at a viewing distance of 166.7 mm. That’s much more manageable.
Another option you should consider is using a tool you likely already own: An SLR. The SLR can also serve as your light meter, assuming that it is so equipped.
If you stated the formats and lenses you plan on using, we can give you a table of equivalent view angles.
Here’s an example using a 35 mm SLR for the preview. Suppose that you want to use a 4” x 5” camera and might use 75 mm, 90 mm, 135 mm, 150 mm, 210 mm, 300 mm lenses. It would be most convenient to mount a zoom lens with a wide range of marked focal lengths onto the SLR. You could then read the approximate focal length at the setting that gives you the angle view that looks best in the finder. Of course, you must choose either the major or the minor dimension as your reference.
Since focal length equivalence can only be established for one pair of dimensions at a time for different aspect ratios, you’d need to decide which pair is most important: major, minor, or diagonal. For landscape photography, this is most often the major dimension of the format.
A list of common 4” x 5” focal lengths, view angle across the minor dimension, and the equivalent 35 mm format lens that sees the same angle across its minor dimension is given followed by the long-dimension AOV and 35 mm formal equivalent lens in parentheses.
75 mm, 64.7º, 18.9 mm, (77.3º, 22.5 mm)
90 mm, 55.6º, 22.7 mm, (64.7º, 27.0 mm)
135 mm, 38.8º, 34.1 mm, (47.92º, 40.5 mm)
150 mm, 35.1º, 37.9 mm, (43.6º, 45 mm)
210 mm, 25.5º, 53 mm, (31.9º, 63.0 mm)
300 mm, 18º, 75.8 mm, (22.6º, 90.0 mm)
This could also be set up for a non-film SLR whose format dimensions are known.
As you can see by reading the responses, there are several options to determine the LF lens you need to match the most important dimension that matches the preview. I like using the SLR method for its convenience.
If you need help in determining the equivalences for your format and range of lenses, just ask here. One of us can give you the answers you seek, whether you choose to make a viewing card, multiple cards, or use an SLR. We’d need to know the format and the focal length of lenses you plan to use.