It makes more sense to let the HN community itself decide who it chooses to memorialize. Setting the threshold sufficiently high (e.g. 3/4 or 4/5) preserves the implicit intention that the bar not show up every time some schmuck we've heard about kicks the bucket while bypassing the criticism that memorialization is an arbitrary thing imposed by admins.
Whether the threshold value should be a ratio of for/against votes or for vs. total possible votes (counting abstentions against) is debatable.