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Hi Ian,
If you are just trying to defocus for the purposes of checking the scopes collimation, anything over 10X per inch of aperture is more than enough. If you try to go too high under poor seeing conditions the fressnel rings just go to mush and you defeat the purpose of what you are trying to do. When you defocus for the purposes of checking collimation it is important to go both inside and outside focus and check for consistency on both sides of focus.
When you are star testing the telescope for the purpose of evaluating the quality of the optics, you should use much higher power. Something in the order of 25X per inch of aperture is appropriate. Importantly, you need superb seeing conditions, a properly cooled telescope and a good quality eyepiece to star test it for optical quality.
Cheers,
John B
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