The slider file is 1e29e8e7_manila, you have to edit any one that you want to use unless the creator has already created it for a sprint manila. YOu can use chainfire's gui editor (found on XDA here
http://forum.xda-developers.com/showthread.php?t=470798) to extract the png, then use a graphic editor to move the slider over and down and import the png back into the manila file using chainfire's program.. And you've got a centered slider.