How Exactly Does Multicast Work
Multicast can stack up to 5 times, with a -60% decrease to each subsequent roll.
Meaning, with 100% multicast you always cast twice, and have a 40% chance to triple cast, which then gets reduced by 60% again for a 16% chance to quad cast and a 6.4% chance to quintuple cast.
With say 400% multicast, you have 400% to double cast, 160% to triple cast, 64% chance to quad cast and a 25.6% chance to quintuple cast.
So while there are diminishing returns, it does stack and keep increasing dps a lot to go beyond 100%.
For crit, I don’t believe there is a chance to multicrit just yet, maybe the devs will implement that as well, but I’m not sure. For now, I don’t believe there is any benefit to get over 100% crit (And also often you get the dazed effect on enemies, which means you can easily overcap on crit even with just 40% crit, so you don’t need that much crit at all).
Multicast is very worth stacking higher than 100%, even above 200% (Combined global multicast and on skill multicast), It has diminishing returns, but even up to 500% multicast it remains a very good upgrade as your quad cast and quintuple cast do still get noticeably upgraded.
At the early game, +60% legendary multicast on a good skill is the most powerful single skill upgrade you can find, but late game it becomes more in line with a regular damage upgrade instead.
However, damage upgrades also have a major diminishing return as well, as +10% when you have 100% damage is way better than +10% after you have 400% damage.
Upgrades are no longer multiplicative with themselves, so the large multicast granting upgrades remain competitive later on, even when on high number above 400%.
Going from 400% to 500% still gives you +16% to quad cast on all skills and +6.4% chance to quintuple cast.
Considering that each extra cast is +100% additive damage, it is definitely still strong, especially as more casts also stack more DoT’s, so it scales better than you expect.