Your advice seems optimized for... well, adults. Sure, if you can use your income as a metric, or your expenses, or some other way of measuring your financial situation - great.
Donating a fixed percentage of your income, then doing as you normally would if you had only gotten this reduced income in the first place, can apply to almost everyone. (People like you who don't have an income have other problems.)
Saving is a very good idea - actually, my idea of "financial security" is "being able to save money". It's just not compatible with the "starving undergrad" model. I'm not a responsible adult, I'm a crazy kid. I don't do responsibility. Sure, when I get a job (and it'll likely be high-paying) I'll start saving (also a fixed percentage, at least until I get the hang of things).
Prioritizing saving and vaguely defined necessary expenses... that's what I'm afraid of. If I say "I'll wait until I have enough savings to live on for six months", I'll just spend a little more every month, notice I haven't saved enough yet, and wait a little more. If I say "I'll wait until I can reliably pay the bills and track spending and get necessities like insurance", I'll just keep moving the goalposts - I'll start thinking I need a car, and I might get sick so I need to save more, and I'll have a big crash and drop accounting for six months.
Related to: People who want to save the world
I have recently been diagnosed with cancer, for which I am currently being treated with good prognosis. I've been reevaluating my life plans and priorities in response. To be clear, I estimate that the cancer is responsible for much less than half the total danger to my life. The universals - X-risks, diseases I don't have yet, traffic accidents, etc. - are worse.
I would like to affirm my desire to Save Myself (and Save The World For Myself). Saving the world is a prerequisite simply because the world is in danger. I believe my values are well aligned with those of the LW community; wanting to Save The World is a good applause light but I believe most people want to do so for selfish reasons.
I would also like to ask LW members: why do you prefer to contribute (in part) towards humankind-wide X-risk problems rather than more narrow but personally important issues? How do you determine the time- and risk- tradeoffs between things like saving money for healthcare, and investing money in preventing an unfriendly AI FOOM?
It is common advice here to focus on earning money and donating it to research, rather than donating in kind. How do you decide what portion of income to donate to SIAI, which to SENS, and which to keep as money for purely personal problems that others won't invest in? There's no conceptual difficulty here, but I have no idea how to quantify the risks involved.