[Michael Thayne]

Note: my hunch is that the Pyramid 3/79 is close to accurate if you ditch the term for atmospheric pressure. That would mean, however, that a three-engine NTR would need close to 15 mps delta-V to get off Earth's surface. This is barely possible for an unstreamlined craft that only carries its pilot (if you use the Smaller Systems rule to use a small control room), but that's probably only appropriate for lifting off a coincidentally Earth-sized planet with no atmosphere.