Using window resize to calculate new value of element offset

I have created the following page which Ive set up as 3 tiers controlling the height and using the offset method to animate each tier up and down. My problem is however when I resize the window, this knocks off each of the calculations when going from tier2 to tier 3 which makes me think the offset isnt being calculated correctly. Ive set up a window.resize but at the moment Im unsure on how to correct this problem.

I guess its a matter of constantly working out what the is in the window resize and working it out from there but Im not entirely sure of the approach

Can anyone offer any advice?