Because Blizzard is your primary damage source, you want it to be as powerful as it can be.
When you’re running Frozen Orb with another element, Frozen Orb is typically the alternative for the other main element, and those extra 10 points make that other element more powerful.
you want it to get to -130 on your cold mastery. this will bring most (if not all) monsters to 0 resist. its really up to you tho. of course it wont ever break cold immune. i normally aim for around -130 depending on what other skills i want to work on. if u have spare skills u can sink them in, resist = damage reduction. 75 resist = 25 %damage. 0 resist = 100% damage. -50 resist = 150% damage. of course these are capped on players at -100 and +95 depending on gear. monsters i think are capped at -100 but can stack well over +100. Greater then +100 resist = Immune.
So, for monsters with no resistance, you get no benefit beyond 17 points (100%)
For monsters with 25% resistance (NM Meph) the “cap” is 22
For monsters with 33% resistance (Embalmed in Ancient Tunnels) the “cap” is 24
For monsters with 50% resistance (NM and Hell Baal, cows) the “cap” is 27
For monsters with 75% resistance (Hell Meph) the “cap” is 32