Ah.
The primary reason for me doing so is because many of my calculations over the past years, before I actually got access to the game's files, circulated around observing the Imp. It seemed to be the easiest to pin, to determine how much damage it'd take to stagger a monster, or to determine other weapon damage values. I found, in proportion to the Imp, many other monsters were buffed
greatly, and I found that plenty of my original assumptions about monster stats (when it came to starting health values and stagger health thresholds) were pretty close to the actual values in-game.
I settled on 40% because that was easier to manipulate than, say, 20%. At 20% of their original values, the weapons would've been too weak compared to their vanilla counterparts, and would've been subject to much rounding to the point where, for example, damage bonuses from Power Shot or certain headshot damage factors would've made 0 difference. Also, I noticed that many of the weapons were still quite powerful despite the larger health values of several of the monsters. It struck the perfect balance between power, vanilla values, and ammo consumption (considering you don't carry as much ammo as you used to).
Basically, if I did more than 40%, the Heavy Assault Rifle would've been capable of one-shotting Zombiemen and Shotgunners, and not leave them open for staggering. If I settled with less than 40%, it would've made many of the monsters still feel spongy despite any upgrades that would otherwise mitigate said situation. I found it easier to stagger and not
kill monsters with such values, especially for the tankier ones.
Part of the reason I'd do it this way, which led to another observation, was that they gave many of the monsters larger health pools not just to compensate for the increased damage of your weaponry, but also so they could be more easily staggered, for you to better set 'em up for a Glory Kill.
Another reason I settled on 40% was actually because in my first machinations about making a prototype version of EOA Monsters (which would attempt to recreate the 2016 monsters in GZDoom), I considered using the damage values
straight from the files themselves, and globally compensate for such ridiculous damage values by globally increasing vanilla actors' max health by 167%. However, I found that if I reduced the 2016 Imp's health to 40%, it'd be left with 60 health (the same starting health as the vanilla Imp)... meaning that all this time, the
vanilla Imp had been the perfect monster all along to test the damage values I had thus far, in a way that'd be more faithful to the game, while also not requiring me to globally change health values for vanilla actors. Also, I found that globally increasing all of those monster health values by 167%, again, got me
close to many of the actual in-game health values of the 2016 monsters. With that said, vanilla Pinkies began acting as good targets to test out firearms against tanky fodder such as Soldiers, and vanilla Barons were great to test out firearms against, as though they were Mancubi from 2016.
By the way, I reduced these values
to 40% of how they manifested in-game.
I'd write a TL;DR... but it wouldn't do this explanation justice. I hope this helps you understand the labyrinthian thought processes behind the final values I settled on.