Question regarding elemental penetration: how much is too much?

So how much elem penetration would you say is:

- necessary (to cover your basic needs with minimal inconvenience)
- good to have (to make sure no damage is wasted on resistance in most cases)
- excessive (to the point where it doesn't benefit you anymore, unless penetration can go in negative res, in this case let me know cause I have no idea)

Separately for mapping and for bossing (I assume you need more for bossing)?

Since you can not pen below 0 in poe2, anything above 30-40 seems like a waste. You can use poe2db to see resists on monsters. Generic bosses tend to have 30-40 range, with pinnacles at 75.