Startup capital for forex trading should be based on your Trading Plan. Risk/Reward ratio that your trading logic can work comfortably. And ultimately whats more important is that the returns you would draw out of your system if it would add any value to your life.
Also another point to keep in mind is the broker, leverage they offer and the minimum lot sizes. All these factors should favor your trading system so that it functions smoothly without compromising.
I hope this helps.