It's not really like a definite, this will make Oxy 5 times more potent.
It really depends on the individual and how they metabolize / react to the enzymes in grapefruit juice and how it slows / increases blood levels of oxy.
I would say it can help by maybe 1.5x. The other day I drank grapefruit juice instead of my morning coffee before taking my 100 mg methadone dose and I could feel / think I could tell I was a bit higher than normal. But I couldn't be positive it wasn't overwhelmingly better, so I would say it helps, but not by some crazy amount