I have the ground glass with my STC and use an IQ180. However, I don't use the ground glass for critical focusing, I use it for framing. When stitching two images together, it is the only way to see the whole image before you shoot. Also, for me there is some compositional benefit to seeing the image upside down and backwards. I have three masks: 54x40, 76x54 and 90x40. Those last two represent a vertical stitch and a horizontal stitch 18mm each way. In my opinion a geared head like the Arca Swiss Cube or D4 makes this much easier since you can adjust in one direction at a time.
I have a Leica Disto for critical focusing. In general, if I am not using tilt and shooting at f/11 or smaller, I can use the GG:
- Install the correct mask
- Mount the lens
- Mount the Alpa loupe
- Open the shutter
- Open the lens aperture wide open
- Frame using the A/S Cube
- Focus
- Mount the back
If there is a single-distance subject at f/11 or larger, I will pull out the Disto and set the HPF ring. That is always accurate. If I am using tilt, I will try first using the GG. But often I cannot get sharp enough so I use a tilt calculator combined with the Disto. I keep trying manually first so I learn and improve. I have two loupes that I carry: the Alpa 3x (310.010.040), and a std Hoodman loupe. The Alpa is the better experience, especially when framing a stitched image and focusing but attaching and removing is a bit fiddly. The Hoodman is quick for a single 50x40 frame. I've tried using a 10x loupe for focusing, but I find it faster to just pull out the Disto and I have not found an acceptable method to shade the GG when using a bare loupe.
There are multiple ways to frame and focus these things. Live view, Disto, GG, Surface Pro 3, estimate, ... you just have to experiment and see which one fits your style and what you are willing to carry. Google Wayne Fox's technique for using the IQ180's live view w/ variable ND filter. Also Ken Doo's work on using a Surface Pro 3 running Capture One.
Dave