I wouldn't necessarily call it a business plan, but I have thought about how much I could make and how soon. I have decent emergency fund, too.
Basically, I should be able to charge $50+ / hr or close to that on a project basis. In my contract job I was paid $37 / hr and was probably billed at double that for what was mostly administrative work, whereas most of my experience is real analytical work. Point being, even just 50 hours per month at $50 / hr = $2500 pretax, which would still mostly cover my main expenses, and I expect to get at least that much work at that rate or better. In the worst case scenario, I'm in contention for a decent full-time job as well as a staffing agency that can give me part-time temporary work.
I'm not really trying to start my own company, as much as I'm just looking to find other entrepreneurs that I can support. I have an aunt with a CPA, which certainly helps, though. I agree and know I should have a separate bank account for business purposes and find a labor/self-employment-focused lawyer, as well.