Different people have different approaches to this, which I respect. For me personally, quantifying things has been a big help. While there are many variables, several of them lend themselves to be quantified and doing this has helped me get more consistent results.
With respect to guiding - at the first cut, as long as guiding RMS is better than your image scale, you're better off worrying about other issues. Today, I image at 480mm with an ASI1600, which is 1.6"/pixel. If I get 1.1" RMS on a bad day, I still shouldn't be focusing on guiding - I won't really see the improvement in my images. But I am soon migrating to a 10" Newtonian which has a focal length of 1000 mm. That's an image scale of less than 0.8"/pixel, and I can pretty much guarantee that if I get 1.1" RMS guiding, I am going to, at best, lose resolution, and at worst, get egg shaped stars if guiding in one axis is worse than the other. That would make me sad, One approach for me is to buy a mount that costs 3x as much as my GM811G and be guaranteed of great results at that focal length, but the other is to see if I can improve the current equipment's results with some effort. Hence my interest in whether Richard's number is 0.3" RMS or 0.3 pixels. If I know that my mount is capable of consistently being guided to, say, 0.6-0.7" RMS, then that would justify me putting continued effort into improving guiding. Other imagers I work with do get these numbers from where I am, but they also use much more expensive mounts.
