>>63922621The shoulder width is the fastest and most simple to use because it's part of the drop compensation hold. You can extrapolate target width if it's rotated to some extent too with just a little bit of thinking.
Meanwhile full height stadia like the one you posted is virtually the worst kind there is since it works exclusively on targets standing in the open and not laying, kneeling, sitting, standing in tall grass or behind obstacles, making it nearly useless for actual combat. Might as well use the mildot for ranging instead.
Head size stadia is also ass on smaller magnification scopes because the object is so small at the ranges you would want to use the rangefinder on.
US used 18" target size in Vietnam and for some time afterwards, which corresponds to the torso height from the belt buckle to the chin strap of an average human. It's also what hunting rangefinders were using for game animals. Although I think it's an ok solution the need to measure just the torso slows your ranging and makes it less consistent and the 36-40" or 1 meter is ideal, as it works for the height from the groin to the top of the head, shoulder width as well as numerous other objects like wheels, doorways, even guns themselves and so on.