What I did: I first identified my favorite places in the world to live: San Francisco, Honolulu, Paris, Amsterdam, New York City, and Lake Tahoe. I then looked up the median rent and housing prices for each city. Then I factored in private education costs for two kids to be conservative given I may not have two kids and public schools are often good enough. After calculating all vital costs, I then did a self-assessment of how happy I was making $50,000, $100,000, $150,000, $200,000, $250,000, $350,000, $500,000, and $750,000. I decided working 20 hours a week making $200,000 a year is the best income balance for maximum happiness. 
In addition to an extra income stream, residual income allows you to diversify your revenue sources. Instead of relying on your standard paycheck, you’re earning money through the royalties of an eBook or the sales of an online course. Diversity of income gives your overall financial portfolio more security and depth. Similar to passive income, residual income gives you more financial stability, flexibility with your lifestyle, additional retirement savings, and a more robust financial standing.
Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.