Be sure to include the flange-to-sensor distance of the camera (often around 17.5mm), and any OAG thickness, or filter/filter wheel when setting the back focus of the reducer. Remember to also add one to two millimeters (remember what you learned about the refractive index of glass back in high school?) for any glass filter that you have in the path.
Many filters are around 3mm thick and refractive index of glass is about 1.5, so a 3mm thick filter will increase the optical distance by 1.5*3mm - 1.0*3mm = 1.5mm. Don't try and measure the thickness of the filter yourself, you will scratch it -- just look up the manufacturer's specs.
Right offhand, I would trust the focal length obtained from the ASIAIR plate solving.
You will need to fine tune the back-focus eventually anyway (instead of using 105mm) to get the best flatness for your sensor size. The optimal back-focus will give a different equivalent focal length.
If you distrusts the focal length that is computed by ASIAIR, save an image that has perhaps 100 visible stars to a PNG, JPEG or FITS file, and then upload the file to the astrometry.net page here.
http://nova.astrometry.net/upload
Be sure to save the file using ASIAIR instead of taking a photo of your screen. You probably already have such a file on your computer somewhere that was taken after the last time you adjusted the back-focus.
In less than 10 minutes, (usually in a minute or so), you will get what astrometry.net computes. It will give you a plate scale (arc seconds per pixel) in the results. From there, you can compute the focal length of the imaging train, given the pixel pitch (ZWO publishes it as micro meters per pixel) of your sensor.
Clear skies,
Chen
