You are basing that on IPP = 88.2% x Sync.
But you don't do inverse by adding 100% - 88.2% to IPP. You need to divide IPP by 88.2%.
Therefore you need to add 13.4% to IPP.
However, while this is true for WBC, is this also true for FTTC?
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU => 2010: Orange 16 Meg LLU => 2011: Orange 19 Meg WBC