I may have not explained myself properly. I understand those are the key "items" to make a very creative and dynamic encounter. My question was, once you have all the ideas and mechanics in mind, how do you go about putting NUMBERS down on enemy sheets, cause it's not as simple as doing it in an "empty room" environment, where you can average rolls and mathematically pin-point the difficulty of the encounter.

I think both approaches are good for different type of mechanical avenues.

I prefer "Roll then Go" for combat, as you try your best to fight and spend whatever amount of successes you gathered for whatever result you deem best in the situation, avoiding "you just missed" scenarios where you fail cause you wanted a very difficult, specific outcome and the DC was set high accordingly. Same for investigation and perception, cause I want to know what your character, with its qualities, makes of the situation, not what you make of it, this is not a puzzle game, your wits can't surpass your character's.

On the other hand, I prefer "Go then Roll" for social tests, cause rolling first just breaks conversation flow, so we use the roll to determine how convincing your character was; his body language, eye contact, demeanor, cadence; these are all things that, altho you can be a great actor and do perfectly, still come down to your character's charisma, not yours; same goes for outside influence, like horror stuff, as it might not scare you, the player.