You will not evade me, $name

[setver]

MOST, not all, mobs 51-52+ summoned. They definitely didn't want all mobs to summon, so there were special cases in almost every zone.

I can't recall any mobs summoning after 1HP, but I can't be 100% sure it didn't happen. The rest definitely started to summon at around 98-98.5% I'd say. It was probably 98.5 and someone would hit it hard and make it drop to somewhere in the 97% area. Some mobs of the same name also did not always summon. I recall this from GoD directly, not sure if it applies to earlier mobs. Those mobs were clearly above 60, so they might have raised the summoning threshold on each expansion.

I didn't ever time it, but it seems the refresh on the summon was 8-10 seconds.

This is from my experiences raiding and soloing as a druid starting late velious up through GoD. It could have been changed from 1999.