There is no unique answer to this question, but I'd say, at least for EK's, the best choice would be:
Your own spells: 100%
Boss spells: 70%
Creature spells: 50%
Other player spells: 10%
Why do I choose this route?
EK spells usually aren't very flashy, so it is very likely for you to miss your own spells when the screen is too polluted.
Bosses tend to spam lots of spells per second, so it can get really annoying at 100%, but those spells generally are useful to know what the heck is hitting you, so 70% should be strong enough for you to see every one of them but no too fierce so it gets hard to tell what's happening.
Creatures do not spam like bosses, but usually there are too many on the screen, specially if you're team hunting, so I like to make their spells much more subtle to see, but still strong enough so I can see peaks of power. This is why I choose 50%, but I guess from 30 to 50 would be rather similar in this case.
As for players, since I play on opt pvp worlds, I really do not need to see their spells, so I opt to put at 10% so if I want to, I can make an effort and see some spells flying over the screen, but usually are totally ignorable.
That's my configuration for opacity spells, but take in mind that what suits for me might not suits for you
