If you can open your practice with minimal expenses, you could associate 2 days a week and be in your practice 3-4 days (including Sat. sucks, but your just starting off, can't have the whole cake the first day). If you can do $2000 a day associating that is $5600 a month take home. Now, if you can do $1000 a day in your office (on average, obviously, some days you will do more starting out, some days you will do nothing) and with 70% overhead (which is way, way high, if you can keep your startup costs under $100,000 as someone suggested your overhead would probably be around 50%), you will take home an additional $7200 a month. So, with your associate job and 70% overhead in your own practice, you will be living on $12,800 a month. Take out taxes, student loans, and retirement, and you will probably be around $4,000 a month cash to live on. Not that bad. You can live a good life on that.
Now, that would just be the first year. They say your income would go up 10% every year. Figure that in, and in year two you a lot more money to do what you want to, year three more etc. Technically your income would increase more than 10% too, because your retirement amount, student loan amount, office loans do not increase from year to year, so you fun mon would go directly to you and not to those expenses (unless you wanted to pay them off early).
So, having cash flow is very, very important, and as someone said, if you are not a strong leader, scratch might not be for you. BUT if you are smart, work hard, and have vision, it is very doable with the decent lifestyle.