When shooting a full-length portrait with a lens with an actual focal length of 400MM, how far is the lens from the model?

It's just a simple measurement. When the format of 135 is converted to 400mm, the picture span up and down the viewfinder is about 0.38m (horizontal composition) at a distance of 7 m. Because the viewing angle of 400mm is fixed and the picture span is in a fixed proportion to the shooting distance, we shoot at a distance of 3 1.5m according to the normal height.

The one upstairs uses vertical composition, and the calculation result is correct, but I just don't understand how to calculate the object distance with convex lens imaging formula without actual distance, otherwise it is calculated according to lens perspective.