Hi,
You need to scale your crosshair object much larger and also instead of viz.monitor.width/height you need to do viz.MainWindow.getSize( viz.WINDOW_ORTHO ) and since you're in stereo you need to multiply the width by .25 not .5. Alternatively, you can link it to viz.CenterCenter.
	Code:
	import viz
viz.go(viz.FULLSCREEN)
viz.add('gallery.ive')
crosshair = viz.addTexQuad(viz.ORTHO,texture=viz.add('crosshair.png'))
crosshair.scale(100,100,1)
w,h = viz.MainWindow.getSize( viz.WINDOW_ORTHO ) 
crosshair.setPosition([0.25*w,0.5*h,0])
slider = viz.addSlider(pos=(0.5,0.1,0))
slider.set(0.5)
def SetOffset(pos):
	offset = vizmat.Interpolate(-400,-100,pos)
	viz.MainWindow.setOrthoOffset( offset )
	print offset
vizact.onslider(slider,SetOffset)
import nvis
hmd = nvis.nvisorSX111()
 Best,
Masaki