Connor, for answers regarding FRC diving I guess science is just following you and the others doing it. There is no scientific studies on this topic (as far as I know). It is difficult to perform these studies as there are so few divers performing this type of diving (at least at a convenient distance from a suitable lab doing apnea research).
So 60 per cent might be the magic number.
However, the difference between the diving responses initiated at 60% and 85/100% were small (in conditions resembling static apnea). I don't think it could be the only factor explaining your experiences. At least, I will be surprised if that eventually turns out to be the only or major factor.
I find it difficult to understand this from a gas exchange/gas storage point of view. The following calculation is a major simplification, but I think it illustrates what I want to say (values are to some extent based on observations/measurements). Lets say you have a diver with a TLC of 10 liters (RV 2 L + VC 8 L). Diving at full lungs (assume 15% O2 in lung gas) gives you a total lung O2 store of 1.5 liters. Add a blood O2 store of 0.9 liters for a total of 2.4 liters of O2 in lungs and blood.
Now assume the same diver inhaling 60% of VC, that is 6.8 liters in total (RV 2 L + 60%VC 4.8 L), with 15% O2 in the lung gas. This gives you a lung O2 store of 1.02 liters. Add the blood O2 store, 0.9 liters, for a total of 1.92 liters of O2 in lungs and blood.
So, with 100% VC you have 2.4 L of O2 and with 60% of VC you have 1.92 L of O2 (a 0.48 L difference). Not all is usable, but lets ignore that for the sake of making the argument easier.
Finally, assume you have an O2 consumption of 0.25 L/min (a low O2 consumption; I have not recorded a lower average than 0.28 L/min in my studies). This is with a diving response initiated. The time that the O2 stores can support aerobic metabolism in the two cases above is 9.6 min (100% VC) and 7.7 min (60% VC), nearly a 2 min difference. Connor, you say you can dive longer at FRC that with full lungs. I can't explain that.
To get the same time from 60% VC as with 100% VC, the O2 consumption would have to be reduced to 0.2 L/min at 60% VC. The lowest value I ever saw during apnea is 0.21 L/min, if I remember it correctly. Maybe a further reduction is possible, but to explain longer times at FRC than full lungs, I guess you need something else as well.
Of course, such things as buoyancy, relaxation, etc, may be involved.
FRC theory says taking down more co2 helps the dive reflex, especially vasoconstriction
But at the same time, FRC diving will also cause hypoxia faster, and hypoxia definately augments the diving response. So, how can you/we say if it is hypercapnia or hypoxia that is the factor at play here? (Hypoxia will not be a factor in deep diving, with the increase in PO2 at depth, but possibly in dynamic or static.)
Discussing this makes me realize that this probably requires some more studies. Your experiences (that hypercapnia augments the diving response) are not in line with the previous scientific studies I based my arguments on. Of course, I'm also curious to find out why that is so. I'll try to do a study on this.
On your previous question about why assume people with differing co2 trigger levels might respond differently to mild hyperventilation, I'm not really assuming anything. Its just my observation that some people are much more susceptible to B0 than others and I'd like to know why.
This characteristic (susceptibility to low O2) probably follows a normal distribution, just as most physiological variables, and there are many contributing factors.
/Johan