Not quite Tom. They're related to the declared size of the main panel bitmap and the screen ratio. So; if you have an aircraft with a 1980x1080 main bitmap and an aircraft with a 1024x768 main bitmap running on a 16:10 monitor, placing a gauge at 100,100 is going to end up in relatively different places on each panel (I'm ignoring the awful stretch that will result on the 4:3 panel!)
For example: my current project uses a 1980x1080 panel bitmap. The declared size of that bitmap in the panel.cfg ('Window01', entry 'size_mm=') is 960,740 (don't ask - can't remember why). I left myself a note at the top of the panel.cfg file that reads 'x=50, y=66'. So, in Photoshop, if a gauge sits on the main bitmap at 100,100, in the panel.cfg file it has to be at 50,66.
1980x1080 / 2 = 960,740 (almost 50% width, 66% height)
screen ratio is 16:10 (close enough to two thirds as above)
50% of 100 = 50
66% of 100 = 66
I did think about writing this up but it quickly gets to be a bloody deep rabbit hole. It's simple enough if your gauges and main bitmap are 1:1 (as the default panels and gauges are), but as soon as you wander out of that, there is no simple formula to calculate on-screen gauge size (dx, dy). Again, for example, most of my gauge faces are twice the size of the relative on-screen display and some labels are eight times the size to force 4K monitors to display text cleanly. Taking a gauge that sits at 100,100 on a 16:10 screen with a 1980x1080 bitmap declared as 960,740:
1. A 100x100 gauge would be 50,66,50,66
2. A 200x200 gauge would be 50,66,25,33
3. A 400x400 gauge would be 50,66,12,16
For the OP: if you're using default gauges then Photoshop is your friend. Load the panel bitmap and paste the replacement gauges on it as they are a 1:1 ratio and what you see in Photoshop is what you need to enter in the panel.cfg file.