A PM defined the formula (CPI x SPI) with the following explanation below.  I am not sure I agree with this, please let me know if you have any input. 

Cost Performance Index times Schedule Performance Index (CPI x SPI). CSI measures the likelihood of recovery for project that is late and/or over budget.  The closer the index is to 1, the more likely the project’s can be recovered from it’s deviation to the original baseline.