Actually, I try to avoid solo diving, and I teach that too. But I'm also not disillusioned that people will not do it just because I say so, so I would like to give them as much advice as I can. But at least when spearfishing, even if it is with a buddy, it's not really "buddying", ie. he is not actively watching me, so I'm under no illusion that he could actually do much else than retrieve a corpse.
So when I do that, I just dive with a huge safety margin. While fishing, I rarely go deeper than 8 meters and mostly it's even less then that ("anxiety zone" I would rate at around 30m). I pretty much come up after I get the first signs of the breathing reflex and have an alarm set at 1:45, after which I will go up even if there is amazing action just in front of me. I keep my surface intervals long (like Bill, without a watch I could not do it...I want to dive after about 30-45 sec recovery, but force my self not to). When surfacing, even from the easiest dive, I focus on good recover breaths (and always dive with the snorkel out of my mouth).
So for me the main thing is basically a huge margin, as objectively judged as possible (ie not how I feel, but what the watch tells me). I know, I know, it's not really freediving if you keep starting at the watch...But damn it, my body just does not tell me to come up after for example 2 minutes.
I would never do 99% max or even 80%...Hell, not even 60% of my max when alone. And I believe I DO know my limits rather well. This is what I meant by having a certain amount of experience and especially experience pushing your self. I know there are divers that do, very smart ones and ones that are still alive, but personally I would not trust my self enough.
So basically, my static result being a pretty constant 7+, I never do aspettos over 2 min (1:45+ascent). CW pb being 50 and "anxiety zone" 30, the deepest solo dive I've done would be around 15m...Etc etc. Dynamic PB 125m and over 3 minutes in dive time, I never push over a minute while swimming and diving in spearfishing...You get the idea. And the cool thing is, at least so far, I have been able to do anything I ever wanted and more. Get the fish, seen some amazing things, and so forth...
Fully aware that this does not make things all safe, but less dangerous at least...