I also am not aware on this math. It seems to use some form of formula that takes mastery number into account.
The only thing I know for sure is the number of time it hits is based on how many spells of that color you are using, so it will deal damage 2 times with 2 colors, 1 with 1, 4 with 4, etc.
The number of gems and current mastery in the color seem to effect the damage output, but we don’t know yet what forumla it is using.
If you keep using the same build for a while, you get a feel for what will and will not chip kill, but currently we don’t have a way to fully math it out.