For a wire-frame finder like the Speed Graphic’s, you use it from the designed eye point: place your eye at the small peephole/rear sight. The frame and peephole are spaced so the frame subtends the same angle of view as the lens. That means the finder geometry already accounts for focal length: shorter lenses need the frame effectively closer to the eye point, longer lenses farther away.

So you generally do not calculate a separate holding distance in use—the camera’s finder is built around one.

These finders are mainly for framing, not for showing perspective changes. Perspective is determined by camera position, not focal length by itself. If you keep the camera in the same place and switch lenses, perspective does not change; only angle of view/cropping changes. What wide and telephoto lenses change is how much of the scene is included.

External finders also have practical limits: they may be only approximate, may include a little extra area for safety, and can be less accurate at close distances because of parallax. But for general composition, using the correct eye position at the peephole is the intended method.