The 'Correct Amount of Backstory' Rule

Have one thing per level, ideally how you ‘got’ that level. No more, no less. No first-level characters with goblins, dragons, a sleep demon, and an evil wizard in their past: pick one. No eighth-level characters who have ‘been around a bit, I dunno, maybe a war with the giants.’

So, for example, when creating a 5th-lv character, here’s their history.

1: home destroyed in a storm, had to travel overland, bandits.

2: found a treasure in a cave, fought an orc shaman about it.

3: took down a thieves guild using magic drugs.

4: temple ruins with a blind devil-worshipping warlock.

5: saved a lizardfolk village from a sahuagin war-band.

Done! That’s a character with a defined history, reasons to know about a handful of things but not everything, and good call for trauma around, say, deep water and weird religions.