Evars weren't designed to be dynamic when evaluated.
Every time the number of icons changes, you want to !SystrayMove $(resolutionx-systraycurrentwidth)/2$ -30
but that won't work unless it's re-evaluated each time.
That is, when litestep reads the step.rc (and its includes)
it will interpret that and remember it as such. You'll even have to set SystrayCurrentWidth 0 before you use it since litestep's just doing its preprocessing as it should. Try
Note the SystrayCheckOnCommands setting.
You'll need that or you will need to load some module that provides an escape sequence for $ and an accompanying bang to parse that escape sequence used in a string (and execute the result).

...back to building my coffin