Ideal Cold Mastery?

Is there an ideal skill investment into Cold Mastery with Sunders? Should it just be maxed out, or is there a point of diminishing returns?

I’m using both Blizzard and Orb, so I can’t max out Blizzard. I’m putting less than max points into Glacial Spike. But it seems like at a certain point it may be better to put points into GSpike to boost Blizzard as opposed to Mastery since you only get 1 point of resistance reduction per skill point beyond the first point.

Seems like relying on gear to reduce resistance is better. So like if I put 10 hard points in, that’s 13 points of resistance reduction, but with +10 to skills it would be 23 points.

Maybe its better to save those points for GSpike and rely on facets and Infinity for reduction. It seems like with Infinity -17% and the -23% from CM and -15% from facets, this gives -55% resistance on immunes. But on non-immunes you have -115, -85, -15 = -215, which puts you beyond the -100 floor on resistance right?

So it seems like going beyond -200 overall resist you start to “waste” some resistance negation. To where the added penalty only applies to Immunes and nothing else. Is this right?