Mk I gunsight

Can somebody help ?. New in the forum ( in any forum for the matter ! ). 70 years of age, unacquainted with PCs etc ( just mails there and back !).
I am the proud owner of a Mk I gunsight. I have read multiple articles abou the technic of using it.
However, I don´know if the sight itself is working properly or else is kaputt. When I turn the dial “·FEET “, does the luminous circle increases or decreases depending on wing span selected on dial mantaining a constant distance of, say, 150 yards ) OR DOES IT REMAIN ALWAYS SAME SIZE WHATEVER ?. If it remains same size, I don´t see the use of the FEET dial. If it doesn´t it means I bought an expensive, deteriorated crap. Help please !