Magnitude of shock - how good is it really

Slightly off. Your lightning damage total with +400% increased lightning damage is actually 500% lightning damage total (Your base is 100% to start). So in this example, for the amount of +% lightning damage you'd need to have an equivalent boost of +100% magnitude of shock is actually +100% lightning damage.

So 100% of this stat gives an extra 20% 'more' damage.

To calculate how much 'more' damage we are getting, we use (new value)/(old value)

so for 100% shock magnitude, we have 1.4/1.2, which is 1.16, or 16% more damage.

if we have no shock magnitude, then 100% shock magnitude is equivalent to 16% more damage.

Is this equivalent to, say, already having 400% increased lightning damage, then adding another 80% lightning damage? This would result in 480/400=1.2 ie 20% 'more' damage

you need a /500 in there, not a /400.

this is because when we have 400% increased damage, we are dealing 500% damage, not just 400%.

with 0% increased damage, we still deal 100%.

so you would need 100% additional increased damage, not 80%.

600/500=1.2

I think magnitude of shock is important particularly for conduit.

if you're not using conduit not sure if you're supposed to care much for shock magnitude?

does magnitude of shock apply to damage? will it increase fire and physical damage as well?

Plasma blast is 80% lightning. Thoughts on if it’s worth it? Shock magnitude doesn’t work on PoB currently.

I think it's worth noting that shock magnitude is a prime candidate for weapon set swaps for bossing.

Most builds can quickly apply a shock with boosted chance skills as fight begins with huge shock magnitude & duration passives and then swap to a dps setup to enjoy high MoS scaling at the same time as other, more consistent forms of damage. I use a passive buffed voltaic mark & exposure as well to get about 78% shock + 25% exposure with minimal dps loss on my primary skills. I'm not even sure if you need to be primarily scaling lightning to do this, since a lot of these nodes are kinda randomly placed and some skills have like 50% or more increased shock chance base.

Obviously this isn't as relevant if you're one banging every boss anyway, but I think this use case is underutilized.

When i went from 20% to 50% shock magnitude, my damage skyrocketed. It makes a huge difference, and is very easy to obtain.

+75% on support gem, +75% on 4 passive points.

Well this is a bit weird question because it's pretty easy to get a lot of magnitude for a few points that is very easily pathed by if you are going by any lightning nodes at all. Or having it has a cheap free option in jewels or other stuff. In an optimized path you start to run out of efficient general damage node. So magnitude is a prime stat to allocate as you get higher and higher level.

Currently running two shocks of 77 percent but thats mainly for lightning conduit 

Depends if you're purely lightning damage or if you have conversions going on. For a skill that's converting 30 percent of phys to shock, magnitude is going to be better. If you're just running spark it's pretty much the same as getting lightning damage. You've asked a pretty nuanced question that depends purely on your build so you'll get a lot of conflicting answers.