I would have to agree, its my understanding your present condition of your body plays a big role as well. Dehydration, lack of sleep, drinking alcohol the night before a dive, etc can make you acceptable to narcosis.
Certainly with me most of my sea dives are after 2-3 hours sleep the night before and that does make a difference. Dehydration is also quite common for me due to the "toilet in a drysuit" issue and i do notice a difference on those rare days i get sleep before a sea dive.
With me ive been (noticeable to myself) narced before now at 25m (82ft) on a particularly bad day. Usually though i get the bells/buzz almost exactly at 32m (105ft) depth which is my sign to start paying a lot more attention to whats doing on.
I've been completely out of it at 35m but at the same site on a different day been relatively ok at 56m(184ft).
The main thing with narcosis isn't how quickly you can write your name backwards but in critical thinking under pressure. You may feel perfectly OK only for it come bite you when something usually simple such as a flooded mask causes big issues to fix with coordination gone and so on which you hadn't noticed until it happened.
Narcosis is generally fine while things are working - it really manifests when a little pressure is applied mentally, anxiety is amplified and so on.
Some examples i can recall from myself. Scenario 1. A wreck dive on a submarine in about 36m(118ft). Going down the line should have seen the wreck at 28m, seabed at 36m. We got to 36m. Common sense would dictate id have worked out the line was no longer on the wreck and we'd missed it. My narced mind was still doing all the normal dive things like checking depth, time, gas, buddy and so on BUT for some reason i thought i was on the wreck. I was thinking "its huge". In reality i was just on mud. So well rehearsed motor tasks were working fine but common sense thinking had gone.
After a few minutes i twigged we weren't on the wreck. Again logic dictates to go back to the line following the scrape marks in the sand and do a circular search as it wasnt more than 20ft off the wreck. My confused mind couldn't grasp that obvious concept and instead wandered aimlessly for another 15 mins before aborting.
It was only on the deco stop reviewing the what went wrong that the obvious following scrape marks, circular search and not realising id missed the wreck became obvious to me - once my head had cleared. At 6m (20ft) it was all so clear what had happened and what i should have done. At 36m it wasnt.
To me at least it showed what narcosis can do - some tasks carry on as normal but other things just dont compute any more.
