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Marc, Alex's answer is mainly the nuts and bolts of what I was going to say or to that effect. To clarify, from my own experience if the star moves just a little at one magnification then the imaging scope needs to move the same distance. If for instance the pixels of each sensor are close then the scale is different and even if PHD can guide to sub pixel this error can be more evident if the focal length is double the size or more. That is because the error is magnified in the larger imaging scale.
Added to this is flexure. Any error is magnified even more with the elements of flexure involved. So instead of the error being double it then becomes triple or more. If you guide with a guide scope you will have some element of flexure.
Guiding can never keep up with that. So that is why I prefer to keep the focal length at least half the distance of the primary.
Mathematically, what Rally says is correct but in practice this is not what really happens.
Hope that makes sense.
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