I am going to disagree with you because the narcotic affect of oxygen does not factor in shallower than about 300 feet sea water. Otherwise all the trimix trained divers would have wasted all that money on learning to and using helium to reduce/remove the narcotic effects from breathing nitrogen at depth. Anyway, who dives deep enough, regularly to get narced on oxygen?
Well Rick already gave the answer I was about to give.
Have you actually checked out any trimix training material?
You'll find that practically all of it uses the (100% - He) rule.
NOAA Diving Manual, Diving for Science and Technology. 4th. National Oceanic and Atmospheric Administration. 2002. "[16.3.1.2.4] ... since oxygen has some narcotic properties, it is appropriate to include the oxygen in the END calculation when using trimixes (Lambersten et al. 1977,1978). The non-helium portion (i.e., the sum of the oxygen and the nitrogen) is to be regarded as having the same narcotic potency as an equivalent partial pressure of nitrogen in air, regardless of the proportions of oxygen and nitrogen."
Some years ago the rule was to consider only the N2 as narcotic but AFAIK current thinking is based on the fact that although oxygen has greater lipid solubility than nitrogen and therefore should be more narcotic (Meyer-Overton correlation), it is likely that some of the oxygen is metabolized, thus reducing its effect to a level similar to that of nitrogen.
You can also check out
Rubicon Research Repository: Item 123456789/2810
"Title: Roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis
Abstract: In an attempt to determine the roles of nitrogen, oxygen, and carbon dioxide in compressed-air narcosis, the effects on performance (mental function and manual dexterity) of adding CO2 in various concentrations to the inspired gas under three different conditions were studied in eight healthy male volunteers. The three conditions were: (1) air breathing at 1.3 ATA; (2) oxygen breathing at 1.7 ATA; and (3) air breathing at 8.0 ATA (same inspired O2 pressure as in (2)). By relating performance to the changes induced in end-tidal (alveolar) gas pressures, and comparing the data from the three conditions, we arrived at the following results and conclusions. A rise in O2 pressure to 1.65 ATA, or in N2 pressure to 6.3 ATA at a constant high PO2 level, caused a significant decrement of 10percent in mental function but no consistent effect on psychomotor function. A rise in end-tidal PCO2 of 10 mmHg caused an impairment of approximately 10percent in both mental and psychomotor functions. The results suggest that, at raised partial pressures, all three gases have narcotic properties,....."
I'm really curious where the 300 FSW figure that you quoted comes from.
