PEMDAS says 15% is the correct bonus.I'm aware of the possibility that it might be 8% due to the wrong sign in the description. The description is crystal clear to me (and many others who tried refining/trading upgraded 3Ds) though, and it should be:
5% * (6/2) = x
5% * (3) = x
15% = x
I'm hoping this is a script bug and not a description bug cause I already burned 6 EC for this lmao.