I don't know where those formulas came from.
But Resonance on all current artifacts is random at the time of spawn.
Personally I find myself altering between fire and physical most of the time.
Casting focus works similarly to resonance except it doesnt care about what type of damage it is. Also casting focus is augmented by Inscription.
So you can get up to 12 from items. and 5 from inscription.
(Formula for casting focus edited in:
Interrupt Chance = 100 -Total Casting focus from items + Inscription bonus. Then random roll from 1-100. If your roll is higher than the interrupt chance, CONGRATS you continue casting)
So 5% casting focus really is 5% Chance to ignore an interruption.
If you run 10 physical resonance, and 12 casting focus. (Since they get factored separately.) thats a 1 in 10 and a 1 in 8 Chance of avoiding interruption.
So if you figure that fighting a dexer if you tried to cast a lev 7 spell, (FS or Spell plague) And you did it right after his last swing.
You have better than a 1 in 8 chance to get that spell off.
Now I don't know about other mages, but with those numbers I'm way more inclined to attempt to fire off a high damage spell when fighting a dexer, than always relying on faster low damage spells combined with poison to take them down.
Also in caster vs caster, I know that with 0 casting focus that when he's at 40 health and I'm at 40 health, and he's firing magic arrows at me, that I have to stick to my faster spells. With my casting focus up or resonance, I may just try to hit with an explosion instead of pew pew pew with magic arrow/harm etc back and forth.
The important thing with alot of the new spells, and properties, is they are not like potions, defence chance etc, the reliable guaranteed known effect stuff. Yeah, it may save your butt allowing you to get a lucky heal off that you otherwise would have been interrupted on, but getting that lucky heal is SO not worth building the suit over because it isnt reliable/dependable.
However if I know that I can take 3 hits before I die, I might be willing to take the risk of getting my big spell off, knowing that if I get fizzled I can recover, and if I dont get fizzled, well I'm not the one who's in trouble.