Hey Bart,
Apologies for the late response. Your question re calculating the FWHMs is a good one, and not an easy one for me to answer--your math is probably as good as mine!
The scopeFWHM will be affected by the aperture of the scope, the quality of the optics, even the size of the pixels. My intention in posting that formula was more that if you plug in some vaguely reasonable estimates you can see how one variable affects the overall picture and then make some reasonable decisions.
Seeing in typical locations in Australia will range in the 1" - 3" so let's use 2" for seeingFWHM.
I have no idea what scope you're using and even if I did there's a lot of variables in scopeFWHM that I don't know how to account for, so let's just plug 1.23" in there (you're sampling rate).
Let's take the aforementioned 0.6" RMS for guiding and then we get:
sqrt(2^2 + 1.2^2 + (2.35 * 0.6)^2) =~ 2.73" FWHM
Now let's see what happens if our guiding RMS jumps to 0.8":
sqrt(2^2 + 1.2^2 + (2.35 * 0.8)^2) =~ 3" FWHM
And dropping the guiding RMS to 0.4" gives us:
sqrt(2^2 + 1.2^2 + (2.35 * 0.4)^2) =~ 2.5" FWHM
This doesn't entirely answer your question which was "How do I calculate how good my guiding needs to be" but it tells you, given all other variables remain the same, the impact that RMS guiding error has on your images, from which you can make a decision on what's good enough for you.
If your image scale is 1.23" and your peak guide error is only 1" then I'd suggest your images are likely to be excellent and I certainly wouldn't go pulling my mount apart if I were you.
Cheers,
Lee