I could not figure out the focal length for my setup. I tried a lot of different values in my ASIAir, but it was never able to complete plate solving because the focal length was off. I even tried using a value of 0, which is what it recommends, but that did not work at first (I later got it to work).
Eventually what I did was upload one of my light frames to astrometry.net. It was able to identify my image and give me some additional details about the image, most important for this was the pixel scale (7.7 arcsec/pixel). Taking that, along with the pixel size of my camera (4.63μm), and I was able to calculate the focal length.
There are a number of formulas out there for finding the pixel scale if you know all the other pieces of information. I knew the pixel scale, but didn’t know focal length. Luckily, the math was pretty easy to switch things around.
The formula is:
(Pixel Size) / (Pixel Scale) * 206.265 = Focal Length
For me it was:
(4.63 μm) / (7.7 arcsec/pixel) * 206.265 = 124
Caveat: When I was doing this calculation I used an image that was set to Bin2. So my image was actually half as big, which means my focal length was half the actual focal length. Instead of 124, it should have been 248. Be careful when using this method.
I got the pixel size from the camera specifications and the pixel scale from astrometry.net.