sort of a bit wary to suggest this, since it is very easy to misinterpret what is going on. However, I find this tool invaluable for setting up the secondary in my Newtonain after major mods.
http://www.iceinspace.com.au/forum/s...ad.php?t=82230
If you do use a sight tube for positioning the secondary, make sure that focuser is set so that your eye is in the plane where the CCD would normally be - the standard texts do not make this point, but the procedure does not work accurately without this at f4.
good luck. regards Ray