Math-wise it looks good... something like:

movearea / 100 * vol%
(movearea = movearea_length - slider_width)

I'm assuming your 156 is accounting for positioning.

I think Brian is dead on about the error. I played with something similar in my RGB theme and ended up scrapping it - it was just too much of a pain in the ass. Instead, I wrote the slider's xpos to a var and saved it so when the theme started/recycled, it'd be where it was left. Of course it can become out of sync if someone changes the volume outside of the slider (that's what I was trying to fix in the first place), but oh well.