Scuba, the reason the old rule says twice your free diving depth--and it is not my rule---it was taught me and it is in old dive manuals --vs as you said 4 or 5 or 6 or whatever times is really very simple to understand. To put it another way never dive deeper than the distance you can swim underwater without having real redundancy (we talking solo here) because that is the distance or depth from which you should be able to safely reach the surface after a problem. When you freedive, as you well know, say you can comfortably freedive 30 feet--well, then that is an overall round trip distance of 60 feet and thus your solo or no redundant equipment max depth is 60 feet--not 90 or 120 or 300. Like I said, it is a rule I mostly follow but it is not my rule. When exceeding that depth--especially solo--then I feel I need real redundancy and that means something other than an octa.