To have confidence in living off trading, I believe one needs to understand how game theory applies to your approach to the market. There is plenty written on this subject, just google it.
Game theory says that your return from trading your capital = %no of wins x % won - %no of losses x %lost. It is clearly better to have big winners and small losers, but not necessarily true that you need more winners than losers if your losses are small. The question of how much capital you need really cannot be answered until you know what your trading system can make in the market consistently, and not just in hot markets as previous posters have said. The trick is not to suffer a debilatating loss of capital through the market hiccups, which is easier said than done.
Everybody will have a different trading style which will have a different formula.
Anyone who has traded the market consistently with the same method for any length of time will intrinsically know what the above formula is for themselves. Very few people would win off every trade. It is simple to look back on the trades for the last 12 months and see what % won and what % lost and how much the winners made and losers lost.
If you break out different types of trades it becomes quite illuminating on your trading habits, and what sort of trades to cut out, or do less.
Speculating is when you know the odds are in your favour, so applying a bit of game theory to your trading results helps understand what it is you do, and where/when to scale up if you want to increase position size.
One of my systems I use has sometimes had ~20 trades in some years and no losses. But Mr Market never stays idle and loves to change, so nothing is ever certain.
