Bruce,
Sorry I forget to mention something else.
Rajah, correctly refers to the offset of the secondary mirror. He has a guess and estimates it to be 4 to 5mm for your scope which was a pretty good guess. In the case of the 10"/F5 GS dobs its actually 3mm. Here is a post I made on the talking telescopes Yahoo Group 4 days ago. This explains a bit about secondary offset and how to calculate it accurately for your telescope.
http://groups.yahoo.com/group/telescopes/message/119450
However, I will conclude by saying that its not necessary with a 10"/F5 dob for visual use, however you can offset the secondary if you wish. At this stage of your collimation learning curve its only likely to confuse you so I wouldnt worry about it.
CS-John B