HEK Rules/Ratings?

[MarkSheppard]

I'm trying to add some superheavy artillery for my own personal OOBs plus some unique mortars, and I'm wondering
what formula the game designers used for HEK....

all I've been able to find is:

Mortars:

All Mortars/Rockets use HE Kill two levels up from their calibre size.

Is there a formula that the designers used to get the HEK
ratings for automatic cannons, machine guns, artillery,
mortars, etc? Because that one snippet in the MOBHACK guide
is confusing.