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Angular size of the image in degree = (size of the sensor or film frame in mm) / (effective focal length in mm) x 57.3
Draw a simple ray diagram if you want to see where this comes from. Note: effective focal length = focal length of the scope x barlow magnification; for your example, with the 2x barlow, that is 2 x scope FL.
If you want a typical eyepiece (say 50 degree field Plossl) power equivalent then divide the apparent field of view of the eyepiece, i.e., 50 degrees, by the above calculated angular size of the image.
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