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I use a ST7 also, on my LX200 (classic), using CCDSoft..
I've also read 3 to 10 second exposure time is optimum. Personally I find that if I get down to 0.5 second exposures with good brighness then I have a dramatic boost in image quality, I believe that it must start to keep up with some atmospheric distortions - but I am not sure.
If viewing conditions are very bad, I sometimes find longer exposures better.
I maximise the calibration distance, but it's usually still fairly short (about 7 seconds).
I disable DEC autoguiding in one direction, such that there is no complication of backlash in DEC. This is not a problem as the star only drifts in one direction in DEC, due to polar alignment error.
I usually give the images a pre-start delay of about 5 auto-guide exposures, allowing the autoguiding to settle down a little.
I use an aggresiveness of about 7 (out of 10) so it makes slightly less of a move than it otherwise would have, so it doesn't over comphensate and get confused.
Hmm, I can't think of anything else right now... Autoguiding certainly can take a bit to get right, especially if you are trying for RA and DEC in all directions and comphensating for backlash in either. I'd suggest starting with RA only.
Roger.
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