Can you provide any rationale for the 3%? Or is the number just taken out of nowhere?

Why not 1%, 5%, 10% or even 20%?

And I must be missing a point, how would the rule prevent the Dallas bitcoinaire from feeling regret for selling 97% of his stash at ridiculously low price? Not to mention that (according to the rule) he vouched to hold the remaining 3% forever, so won't be ever taking any advantage of the price rise, unless he decides not to follow the rule.

For this specific example, the "sell half each time the price doubles" (or variation of it) seems way more appropriate.