itch.io

Glad to hear it, thanks so much! :)

The process for coming up with Threat Levels is based around a few factors.  Each monster was designed to represent the same kind of threat and power level of the equivalent creature from the videogames - as such, the amount of attacks with a particular weapon it should take to kill one (on average) is factored in, as is the monster's lethality, and finally, how difficult it is to escape.

In other words, a monster from the videogame that takes 3 shots from a shotgun to kill has a higher threat level than one that takes less shots, or 3 shots from a handgun (there's more to it than this based on Firearms skills, the habitat they are encountered in factoring in to likely ranges of engagement when fighting, etc, but that's the basic gist). 
Additionally, a monster that can do more damage in less attacks, or one that has a chance of insta-killing a player if they are unfortunate enough will have a higher threat rating than one that can deal out the same damage over a slower period of time.
Finally (but not really finally, as mentioned above), monsters which can pursue players for longer or are harder to simply run away from are more dangerous too.  This is the main reason behind the 'Behaviour' section under each Monster's profile, although it can also serve as a guide for GM's to best capture how the monster acts during a game.

So if you were to homebrew a creature, I would first look at average damage of the best weapon the defeat it, break that into number of shots vs its health.  Then lower or higher that number based on its lethality, actions, traits, and finally its behaviour.  This should give you a ballpark number to work with.
Alternatively, you could look up an existing monster profile that is similar and make the necessary tweaks compared to that.

None of this is an exact science, however.  At the end of the day any monster's true threat will always come down to the player's actions and the luck of the dice! >:)

Thanks for the question!