Is this accurate for a computer and how much so? I got in an argument with a bunch of people who thought that lowering the brightness would lenghten the battery with about double the time which is complete insatt in my book.

You are talking about laptop? Yes, lower brightness will allow longer usage per recharge, double? If really light weight usage it can be close to double in some cases, but in hard usage I would say it can be like 5-20%.

For my school use, this is pretty much true. I have a modern, power efficient processor and most of the time only run office programs. In this situition 50% more battery life should be easy to achieve by going from 100% to 0% brightness, probably even more.