Center GUI per screenwidth

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I'm having an extremely hard time centering a gui control per screenwidth, so when screen width changes it's still centered. I know the GraalControl resize, and I'm using it.

The problem is just finding how to center it. I can get it close to center, but then when I resize Graal's window it gets all screwed up, even with the same center script. I've tried a multitude of things, but this is the most recent....

PHP Code:

x = (screenwidth % getimgwidth("roak_mainmenu_backdrop1.png")) - 230; 


The above is a bit unnecessary, but again, I've tried many things, including just splitting screen width subtracting a number. It just keeps "offsetting" from center as I change Graal's window size.....

Am I missing something?