You'll need an event (in this case from xtray) to control it. $*CurrentWidths$ are static (since you may need dummy variables in an rc file) and need to be reparsed. The xTaskbarX and xTaskbarWidth values in your rc file will essentially be irrelevant. So you'll end up with something like...

Since y and height are static, those can be the actual values. The 148 comes from your button+clock widths. !ParseEvars is from xLabel. And, of course, you'll need a corresponding xTrayOnRemove.