So https://classic.wowhead.com/spell=12985/shatter "Increases the critical strike chance of all your spells against frozen targets by 50%", but it has never been very clear to me if that is additive or multiplicative in effect. My google searching is yielding nothing of use. Hopefully that means it is more straightforward than I thought.

If a mage say had 20% natural crit (made up numbers), does it multiply this by 50% increase to yield 30% crit, or does it just add them together and give them 70% crit?

My heart says it should be 70%, but I'd like a definitive answer.

Having played mage in vanilla I remember it feeling like 70%.

It's definitely additive, and I can confirm it works this way on classic beta as well.