What would be "Proper Jumpscares?"

[Alex Ros]

...Any reason for such a specific "format", btw? ^^

Simplicity. That's the only reason.

...In contrast, I had trouble understanding one of your rules because it was too vague and/or generalizing.

I'd say if that particular rule is not clear enough and needs an explanation, than the rule itself is bad. I think that rule is not something that could be a true rule-law, but just some thoughts... so it's better to delete that rule-law. Seriously. And so far I think that the only really simple and clear (as rules I supposed to be) are only those 3:

1. Do not make jump-scares close to any objective completion or event.
2. Do not make jump-scares that would lead to the player's death.
3. Do not make jump-scares without an explanation in terms of the storyline.