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It doens't matter if your centre dot is out by a mm. It's only there to guide you! Once you have performed the collimation you will soon figure out that the dot "ain't quite right". Make a mask out of a manilla folder, give it three tabs that can sit on the retaining clips so that it doesn't touch the mirror surface. It will look like a trefoil so you can match the circumference of both the mask and the mirror. NOTE: the human eye is very good at centralising, you can collimate a telescope quite well without any tools, close enough that you can finish off with a star test. Other tools are great, but not always necessary. Longer focal ratio telescopes are quite forgiving!
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