Changing value of variable in mc from another swf

Hello everyone, I have one swf file that containes a mc using an easing function to move an arrow around. That mc has the instance named myruler. The arrow moves based on the value of a variable named “y”. This swf also has a mc that I use as a container to load movies into. The movies that are loaded into the container mc have buttons that I would like to use to change the value of the variable “y” in the movie clip named myruler found in my base swf. Did that make sense? The code I’m trying to use on those buttons goes as follows:

_root.myruler.y = 300;

The my ruler movie is in the swf that loads the swf with these buttons. So how do I point to that original swf.myruler.y? Thanks in advance!