Incinerate Stage damage bonus

I asked an even more important question about Incinerate here because there wasn't an answer in this thread at the time. It's fine to answer here or there, but please quote me so I'm notified.

Inspired by this thread, I'm considering to make an Incinerate totem build (because unlike the OP there I think Incinerate seems quite fine, apart from not being a phys spell that can be scaled with stat sticks, but that's a topic to be discussed there and not here).

What I'm wondering is when the damage bonus per stage is applied. It's 25% more damage per stage, but does it start at 0% or 25%, and does it end in 175% (225% with enchant) or 200% (250%)?

And to confirm, the way I understand it (because they should count as different sources for more modifiers, even if they both belong to the gem), the last wave has an additional 500% more multiplier, not 175% (or whatever the number for the question above is) + 500%?

In the other thread I assumed the answers to the two questions were 0% and yes, but for a build I'd like to know in detail (obviously the build will work either way, but more information means more optimal decisions).

Bump. Need an answer for this. I can post in the mechanical questions thread, but my last question there went unanswered while I have answered most questions since then (not necessarily all correctly, but I'm not GGG).