If you don't have any money in the bank, then you shouldn't quit your job. However, I feel that it's better to quit your job after saving money than toil away for a much longer period of time trying to do both.
I suppose it also depends on your startup. If you are trying to start the next zany Twitter or Facebook app or your goal is to just get funding, then yeah, maybe a year of savings won't work...
A friend of mine had a great idea that we are going to work on and I just thought it's now or never. If I try to work and do this on the side, I don't really see myself putting in as much as I could and it would be a regret down the road.
The "To quit or not to quit" question isn't so black and white. Definitely don't quit if you are living paycheck-to-paycheck but if you have at least a year's worth of living expenses saved up, I think it's worth it to give it a shot.
Yes, some people build businesses while they are working full-time. Other people need the motivation or time of giving it their all. It's a case-by-case scenario.
In addition to this, a full work day sucks most of the creative energy out of me. At the point when I would just be getting to my side project, all I wanted to do was take a break, which I pushed through, but my projects suffered.
PS. I like your equity calc.