set your pivot point based on the bbox. alt+t for center pivot. alt+shift for bottom center pivot.
autoPivot
Updated: 18 May 2013
Author: satheesh
Compatible Nuke versions: 5.2 or later
Compatibility: Linux, Mac, Windows
This little script allow user set their pivot point to center of the image or bottom center of the image based on bbox. Must need bbox value this works based on bbox value. Use the sortCut key to set the pivot point.
menuBar = nuke.menu("Nuke")
menuBar.addCommand('Edit/Node/autoPivot/centerPivot', 'autoPivot.centerPivot()', 'alt+t')
menuBar.addCommand('Edit/Node/autoPivot/bottomCenterPivot', 'autoPivot.bottomCenterPivot()', 'alt+shift+t')