$\text{FBI}_{\text{current}}=\dfrac{\text{CBI}_{\text{current}}\times\text{FBI}_{\text{previous}}}{100}$; chaining from 2008 gives the fixed-base series.

The chain base index numbers act as link relatives. The fixed base index (FBI) is built up by

$\text{FBI}_{\text{current}}=\frac{\text{CBI}_{\text{current}}\times \text{FBI}_{\text{previous}}}{100}.$

Taking 2008 as the first year (its chain index $100$ is already relative to 2007):