Sorry - I'll try to clear it up...
IIRC, it was ~110' to the mud and ~90'+ at the deck.
You are correct. The Helium replaces some of the Nitrogen, reducing the narcotic effects of the Nitrogen. Yes. There is very little difference in the narcotic effects of air vs nitrox. The O2 decreases the amount of Nitrogen your body absorbs. There is very little effect on the narcotic effect. The Helium is what changes that.The gas switch at 70' was from 30/30 (30% Helium, 30% Oxygen, the rest - 40% - Nitrogen) to the deco gas, which was 50% Nitrox. The narcosis that hit me was surprisingly strong at only 70', which is a depth most folks say they don't get narced. If I descend to 70' on an EAN or Air mix, I don't generally notice much effect at 70'. It was when switching from the very low narcotic potential of the 30/30 to the higher narcotic potential of the Nitrox that I noticed the effect.
Does that help?