It sounds like you're hardly struggling Paul, but for such a great mount any tracking movement is much more likely to be seeing than mount. So in general, would suggest the following rules of thumb:
- go for longer exposure times (2-5 secs perhaps)
- work out how much guider movement will show on your main camera/OTA and consider adjusting min movement accordingly
- agressiveness of 5 is 50% so tweak up or down depending on seeing
For my trusty HEQ5 which obviously doesn't have anything like your PME for engineering I'm much more inclined to increase the min movement settings and decrease the max move, keep the agreesiveness down (4-5 usually) and find a short FL guidescope to be a big help with seeing (200mm) which is probably along the same lines as Robin describes with binning.
There is a multi-star guiding plug-in for Maxim that theoretically should be the bee-knees, but I haven't been smart to get it going must confess
I've spent many a moonlit night tweaking individual settings back and forth to get a feel for what they do, and think it's all been time well spent getting every last bit of performance out of my rig. Seemed to be much harder to get right under the bonnet like that in PhD, but only my 2 cents.