The concept of using custom indexes to analyze outperformance smacks of the inanity of «returns - based style analysis.» (alephblog.com)
For such a multi-asset portfolio, you could use the 7Twelve Index, calculated by S&P Custom Indices and — full disclosure — based on the model I've created, as a benchmark. (financial-planning.com)
In case of debt or hybrid funds, some institutions, such as CRISIL, create custom indices that they funds can use for the purpose. (unovest.co)