Hmm, well, the actor is in region whenever the ActorEntry even occurs. But I suppose you want him to be in region AND in a specified X coordinate range, right? You could define those StartX and EndX constants as custom properties in SceneEdit (select the region and click the "Custom..." button and add properties named StartX and StartY). That way you would be able to use the same script for all regions. The script would then read the custom properties simply by using "this.StartX" and "this.StartY".
Uff, does it make sense?