IIRC we're talking about it reaching apparent magnitude ~ -12, given or taken a couple magnitudes (or about as bright as the full Moon - Just imagine that much light coming from an infinitely tiny dot instead). BTW with a declination of ~ +7° Betelgeuse is very close to the celestial equator, so the supernova will be visible from anywhere on Earth, save a very small region within 7° from the South Pole.
It's fun to ponder how it will be that bright at every point that is the same distance. Imagine a sphere centered on Betelgeuse with a 600 light year radius with Earth on the surface of this sphere. The fact that enough photons reach my eyeballs to be able to see it at night is difficult to comprehend - the amount of energy needed to sprinkle every square millimeter of a sphere that size is just unimaginable. Now make that 100,000 times brighter during a supernova event -- crazy town. I feel like imagining the energy from a star spread out on a galactic scale helps me understand and appreciate the magnitudes better than any large number of luminosity or watts or photos can.
How much visibility are we talking about?