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I think if you read the links in the link I sent , Stark explains how a star covering many pixels can guide down to a small fraction of a pixel in accuracy with good seeing.
A larger focal distance for guiding is always going to be better which explains why an OAG is always attractive.
The point is also made by Rista that the ratio of the guide scope and imaging scope rig is not really material which is why the figures shown in PHD 2 do not really tell you how good your result will be . As usual the answer is , it depends.
Just better to have as good a resolution for your guide scope set up as possible. Period.
Mine are equal and the idea of a certain number , say 5x, is the upper limit of what will work not a recommendation. I got a 135 mm lense to work but it was not ideal. An OAG will always be better if you can focus and find a star, but a pain on a newt.
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