ActionScript 3.0 :: Screen Resolution And Button Placement

My move is getting dynamically resized to whatever the computer's screen resolution is set. I'm placing a button in the lower left hand corner that always needs to be flush left and bottom. Now on my computer, I can get it there just by putting in
ActionScript Code:
minimizebtn.y=(stage.stageHeight)-220-(minimizebtn.height);

Clearly, this bit of code will only work for my computer's resolution: 1440x900. I've tested it on other machines and if they, too, have 8:5 ratio like mine, they look pretty good. Not so much on a say, 4:3 ratio display. I know I need to get the aspect ratio and then place my minimize btn's y coordinate.

One thing I'm not clear on: why I set my button to x=0; and it will sit flush left, but won't place my button in the right spot with:
ActionScript Code:
(stage.stageHeight)-(minimizebtn.height);
Currently stage.stageWidth=Capabilities.screenResolutionX;
and stage.stageHeight=Capabilities.screenResolutionY;