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Thanks for the insights Alex.
I might be going about this the wrong way, but at the moment I'm not concerned with exactly how far I move each screw. I realise there are recommendations to match the adjustments made to one pair by doing the opposite to the others, but this isn't something I've been doing.
The technique I'm exploring at the moment (and which I haven't had a chance to fully test due to lack of clear skies) is to get the screws 95% tightened at 168x magnification (16mm Nagler), and then add the powermate, push it to 336x for the final tightening. I've noticed that at this high mag - and with the small doughnut you mentioned - the slightest movement has an effect, so my goal is to combine moving the screw the last maybe 1/20th of a turn with the fingers to get that final lock with reaching good collimation.
I'm trying these finger screws so I can avoid fumbling around with the allen key while looking through the eyepiece. You're right that the fine control isn't the same, but it's much easier to feel where each pair is. I've used some moly paste on the threads, but there was issue with the point of contact between the pull screw and visual back. Initially I used flattened spring washers beneath the two nuts in the picture as I didn't have anything else with a small enough diameter to fit inside the countersink hole, but there was still a ridge in the washer that made the final adjustment too rough. I now have suitable flat washers which have solved that issue.
I've kept the original screws, so can revert back if this experiment shows the longer finger screws to be impractical. Your point about the collimation changing each time the original focuser is used is a good one. Using the external focuser goes some way towards removing that as an issue, but I still use the original focuser for the big changes necessary after removing the diagonal to attach a camera. I'll have to remember what you've said about always using the same direction of focus.
It's teaching me a lot about this scope as well as star testing and tuning. Great fun too...
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