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For any given eyepiece design, where the apparent field is not being vignetted by the field stop (or anything else), then it's fairly simple I think.
Magnification = Telescope Aperture / Diameter of Exit Pupil
True Field = Apparent Field / Magnification *
So inserting a 2x barlow, at the appropriate spacing before the eyepiece, is going to halve the exit pupil diameter and therefore the true field.
All that using a barlow lens does is preserve the eye relief, which may make the full field easier to see than with a short FL eyepiece of the same design. The cost is adding more lenses in the light path.
* Wide field eyepiece designs generally cannot maintain a constant magnification across the entire apparent field. This distortion produces a variance to the simple formula for true field size. It is small though, and constant for a given eyepiece design.
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