Most models of type II supernovae expect that the explosion has a peak luminosity of around $10^8$ to $10^9$ solar luminosities. This gives it a peak absolute magnitude of -15.16 to -17.16. At a distance of 200 parsecs, this is an apparent magnitude of -8.652 to -11.15.
Okay, so how bright is that? Well, the full moon has an apparent magnitude usually around -12.75. This means the total luminosity from the explosion of Betelgeuse would be something like $1/1000$ times the luminosity of the full moon. But the human eye sees things logarithmically, so we might expect that it looks like its about a third as bright as the full moon.
This matches up roughly to descriptions of SN 1006 which was a supernova 1,000 years ago. Observers at the time described a slowly growing ball in the sky about a quarter the brightness of the moon.
As for your second question, how we predict the brightness: We have our estimates about the brightness of the supernova mostly from computer simulations. Supernovas occur when large stars run out of fuel and their core collapses, causing all the mass to suddenly fall inward. A series of complicated nuclear reactions take place as the matter rebounds in a huge shockwave. The parameters of this whole process are quite nasty to work out in detail and the results depend somewhat (but less than you might think) upon the size and composition of the star. We've figured out those parameters mostly by running lots and lots of computer simulations with different parameters and figuring out which sets of parameters match up best with observations of actual supernovae.