I have it on very good authority, that the intent in the wording in the RFM is that for AEO operations the figures quoted (10-20 degrees) are a delta, and that for the OEI case (20 degrees) an absolute value.
E.g you are hovering with 5 degrees nose up, on rotation you rotate to 5-15 degrees nose down on the ADI - the actual value dependant on factors such as pax comfort and day/night operations. If the engine fails, you rotate to 20 degrees nose down on the ADI.
20 degrees is a compromise between maximum acceleration to Vtoss and excessive rate of descent.
It is accepted that there may be some ambiguity in the current description and it will be addressed at a future RFM re-write. Hope that clears that up
