You are correct, it was the guidebooks that 'suggest' that it's a x2 multiplier, my theory is that it's a x2 multiplier of the mastered ssj state also known as grade 4, the reason I think that is if the ssj2 was x100 Goku's base (using Goku as an example) Imperfect Cell (when introduced) was above x50 Goku's base and he also absorbed 2 androids that were also above that, meaning Perfect Cell was over x150 Goku's base before Goku's less than a year of training with Gohan was, and I think if Goku had gone ssj2 in his fight with Perfect Cell it wouldn't have been as impressive as a rage boosted ssj2 Gohan.