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First off, don't get fixated on the guiding figures, it's the image results that matter. My recent results have shown this to me - just started using a new mount & while my guiding figures are much worse (I use PHD2), even double what I used to get, my image results are much better even taking much longer subs and the wind blowing.
What was the weather like, any wind around? While the CEM70 is a hefty mount, it can still suffer from the effects of the weather. And this can of course vary from night to night. And the guide speed should be set higher in the mount settings, not lower as that makes the mount 'slower' to react. Try 0.8x or 0.9x instead.
OAG's really aren't that hard, just confusing. Basically, both the imaging camera & the guide camera need to be at the same focus distance. With the Celestron reducer the back focus is 105mm, so both cameras sensors need to be 105mm from the reducer.
Easiest way is to put the OAG in the image train & set the imaging camera to the correct back focus distance and check it can focus correctly. Then measure the distance from the prism in the OAG to the imaging camera sensor - this is the approximate distance the sensor of the guide camera also needs to be from the OAG prism. Add/remove/change extenders as required to get the correct distance for the guide camera. I find it easier to have the OAG closer to the telescope than the imaging camera - put the camera too close & you may not have enough distance for the guide camera to achieve focus.
Cheers,
Mark
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