Take position according to en element

hi everyone,

i have a design of a site: http://www.terranartworks.com/testsite/tsv1/index.html

i’m trying to make:
1- red point will slide and stop at the center position of every button,when i clicked at the button.
2- grey point will slide and stop at the red points position when mouse is not at the buttons area

i try to take an half of width of the link and assign red point to there but it will not working

how can i do this?