Scale is nothing more than a reduction ratio. the door, in your photo measures 1-5/8", 0r 1.625". this represents an 80" door, so....80 divided by 1.625, = 49.23. your door is almost 1/48, or almost O scale.
Now, if you want, you can measure everything on the vertical line of the door, multiply by 49.23, and get the actual size in inches. For example, half the height of the door is .8125". multiply that by 49.23, and you get 39.999375" (40") which is half of 80".
Using an HO scale rule, you measure off the actual number of feet, and/or inches, and that is the HO scale size of the object.
If you do not have a scale rule, and have to use real inches, measure the size in the photo, and multiply by .5658708 to get the actual size in inches of the scale part. 1.625" X .5658708 = .91954", so the door would be .91954" to be an 80" door in HO scale ( WARNING! the .5658708 only applies to the one specific photo, at one specific vertical, and is the ratio of the actual inches, 80 divided by 87(ho scale),.91954, and the actual inches of the photo 1.625" (1-5/8")---.91954 divided by 1.625 = .5658707) as the actual inches of the photo decreases, the multiplier will increase eg. if the photo height measures .8125 (13/16ths), .91954 divide by .8125 = 1.1317415. anything on that vertical would have to be multiplied by the new figure of 1.1317415). HO scale is 1/87.1, 1/87 works just fine...87 X .91954 =79.99998"(back to the 80" door).
Another trick, fasten the photo to a large enough piece of paper, lay a straightedge along the base of the building, and extend the line to the vanishing point (where the lines of perspective meet), then do the same for the top of the wall, to actually set the vanishing point. A line, then drawn from the vanishing point, along the top of the door, will be an 80" height line, on any vertical that intersects it, measured from the base line.
A vertical line along the "tall" edge of the door, can be subdivided into feet and inches, determined by the height of the photo door. The height of any point on any vertical line can then be determined by selecting the desired point, laying the straightedge from that point to the vanishing point, and noting where the straightedge crosses the known vertical of the edge of the door.
Horizontal line measurement gets tricky. One of the lines of perspective that you need to draw, is one that is perpendicular to the vertical.
Let's take a window. You can measure the vertical height fairly accurately, but the horizontal measure is constantly shortening. What usually does not change is the ratio of height to width (as the width is decreasing, so also is the height, by the same amount. So, measure the width, on the line perpendicular to the vertical (you may have to extend verticals below the base line depending on the photo's perspective). Determine how many times that width, goes into the height. If the window measures .5" H on the photo, and the width on the horizontal is .25", then the window is half the width of the height or, 21-3/4" X 43-1/2".
For every horizontal measurement, you will have to draw to verticals, measure the line segment perpendicular to the verticals, and determine its ratio to the vertical, to get the actual feet and inches distance between the verticals.
Confused?? One of the magazines RMC, MR, RMJ, etc. had the mathematics used to determine measurements on perspective photos.
