Question about blood boil

Hi, All !
I have Warlock boiler on ladder, off ladder and offline. Enjoy playing then a lot.
But O still haven't figured out where this damage originates and what is it's actual range.
Range for 40+level

Blood Boil is ~~20 yards. Which seems smaller in fact. I've read it is not radius but diameter. Cuz coders are no good with geometry and other boring school stuff. Anyway it looks way smaller than Necro

Corpse Explosion of same size.
2: origin of the damage : where is the circle center? Should I hover my mouse over myself, over my demons or doesn't matter? Their life is decrease no matter where my mouse is. When I hurl my demons over the wall in WSK and press Boil button it works. No matter where mouse pointer is. Sometimes when demons are next to me melee fighting Mobs demon health is lowered but mobs are still alive. And after me hovering mouse over Mobs or demons it starts working again.

3: if I have 3 demons in different parts of the screen do they make 3 explosions or only one ?

4:overall advises from boilers and dataminers are welcome.