Another point you may want to consider. The orientation of your camera. Assuming your CCD is rectangular (like most) keep in mind it's easier to center the secondary sideways than up or down. What I mean is that you have total control about the centering of the spider square with the focuser but closer or away from the focuser is tricky and that's where most of the secondary tilt will occur once you're collimated. So if you can live with your camera being rotated 90 degrees so the short side of the CCD is parallell to the tube it'l be easier to get good stars on the edges in that configuration. If this makes sense?
