Hi Remi,
You tried to use ForEach / ForAll quoted by Mr. Jesudas?
I do not see another solution, SAP note 2063246 with this open,
awaiting solution. Try something like this:
=((Sum([Measure]) Where([Dimension]=1) ForEach([Dimension])) / (Sum([Measure]) Where([Dimension]=8) ForEach([Dimension])))-1
Thanks