Expression to maintain overall transparancy when blending two layers

Hi all,
I'm hoping to find a mathmatical answer for what I've been trying to do manually..

In a nutshell I have a 5 second fog clip that I need to loop to 25 seconds.. My first attempt was to use several copies and use Keyframe assistant -> Sequence layers to offset them, blending the layers by changing the opacity gently during the overlap. As you may have guessed, it's not a linear thing - ie, the result was that the overall opacity would grow or shrink depending on how the two layers blended.. The visual result was that the fog was getting more dense then thinning out again at each overlap.

Does anyone have an expression to maintain the overall transparancy?
(Or, is there an easier way to extend this 5 second clip without changing the timing of it - I like the way the fog evolves and moves around and don't really want to change that.)


Thanks in advance!

--KTFA