Paul, when determining a line of sight you take objects that line up vertically in the foreground and background and plot them from overhead. From overhead it does matter what height anything is. It also does not matter how long the bike is. you only use a single point like Chaney's helmet and whatever single point is directly above his helmet in the background. If you want to find the location of the headlight you don't need the length of the bike, you just do a line of sight for the headlight itself.