Re: fade in and out without keyframe

Hi All,
long time reader, first time poster.
First of all, thanks so much for all of your helpful thoughts, podcasts and postings. The Cowmmunity has saved my time-strapped rear-end on many occasions.
So, here's my question regarding sliders, expressions and opacity.
I'm trying to do basically the same thing described in this thread, (control many layers' opacity with one layer to avoid many keyframes, etc.)
The only twist being I would like to stagger the fade up of my stack of lights and be able to control and adjust the timing to my whim, (using that all mighty control Null layer hopefully)
I imagine that I just need to incorporate the expression "valueAtTime(time-X)" in there somehow.
My expression chops aren't very developed, but I'm trying.
Thanks again for all your help,
bruce