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I haven't tried that. That method assumes that the top of the OTA has been cut truly square with the axis of the tube. I've been thinking of using a set square - either a small carpenters square, or one that a draftsman would use. Place it with it's longer leg down the OTA, with the short leg on the "axis" of the focuser tube, pointing across the OTA. Look through the collimator, and slide the square up and down. If the focuser is square to the axis of the OTA, you will be sighting exactly along the edge of the short leg. If not, you will never be able to sight parallel to the edge of the short leg of the square.
Of course there is also the other axis of the focuser. Is the focuser's line of sight a true diameter of the OTA?
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