Is this correct? If the Sun is 400,000 times brighter than the full moon am I correct in figuring that one would have to be about 632AU from the Sun for it to appear as bright as the full moon. If the Oort Cloud begins about 1000AU from the Sun, one would have to be more than 60% the way to the Oort Cloud for the Sun to appear as bright as the full moon.
Yes, brightness is inversely proportional to the square of the distance, and the square of 632 is about 400,000. It's probably appropriate to round the value to 1s.f. to give "600AU". Of course, at that distance the sun appears point-like.
The distance to the Oort cloud is rather hard to define. It's not like you reach 1000 AU and see a Star Wars like "Asteroid field" of comet nuclei. For example Wikipedia gives a distance of 2000AU.