No, you can't use the edge of the window itself as the range for determining the depth of field. With that wide angle lens, the gradations between 3 ft and infinity become progressively so small as to be meaningless.

Depth of field, as you know, also varies with focal length. this creates problems with, let's say a 50-300 mm zoom. How do you display that range? With old film era zooms the scale was fan shaped and more or less of the scale was visible because the lens barrel moved in and out with the zoom function. How do yu do that with some of these internal zoom lenses?

The real solution is for the camera manufacturers to make determining Hyperfocal Distance a software function. A calculator could be built in and displayed on the LCD screen. In fact, that wouldn't even be necessary. A simple press of a button would set the focus at the HFD for that lens at the selected zoom and f/stop all based on a generous print size. The camera body already knows what focal length lens is on the camera and can focus the lens to a predetermined distance as these are already a part of the Exif data.

Simple, huh!

The colored-lines aperture scale on older lenses is used to set lens focus to the "hyper-focal distance", which is the lens position that captures "infinity" to near-focus in DOF, depending on the aperture selected. The smaller the aperture (larger number) the greater the DOF. Auto-focus will not achieve maximum hyper-focal DOF.

Basically, you manually set focus by positioning the infinity sign at the farthest colored line that corresponds to your chosen aperture. Every distance to the matching colored line will also be in focus.