Isn't the all engine case sometimes more limiting as you have to add a safety factor to it? (As the all engine case is so likely, and the OEI case very unlikely)
The engine out case can just be the net performance if I remember correctly (and I probably can't).
i.e.
4 engine TOD required = 1000m
3 engine TOD required = 1100m
1,000m x 1.15 = 1150m, more limiting than the engine out case.