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The OAG lets you adjust the focus distance by a small amount, which you tweak with a focusing mask on a bright star.
As for figuring out the focus distance, get your 6D attached to the OAG first. Once you've done that, use a pair of callipers/ruler and measure the distance from the centre of the pick-off mirror to where the camera sits flush to the EOS bayonet and add 44 mm (EOS mount to chip distance). Next, measure out that same distance from the OAG mirror and up the guider mount - this will show you what distance (possibly spacers) you'll need.
I'd recommend attaching the DSLR, OAG, and focus all as close to each other as possible - if you add in long distances such as with the TRUE 2 adapter, you'll substantially increase the moment arm on the stock focuser. It's better to use the beefy extension rings on the telescope side of the focuser to minimise your focuser-to-camera distance.
Good luck!
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