I call it "thinking on your feet". Its by far my favorite way to train. Its just more fun. Just understand different progressions. Like reps, sets, weight or time. Heck sometimes i will even deload so to speak because i'm not feeling it but i don't plan anything. Of course no harm in planning a deload.
SDT - Single, double, triple - progression article on the ground up strength website is a really good read on progressions.
Edit: I never totally understood your post! I definitely keep a keen eye on numbers. But i don't follow strict percentage based programs. Its boring. And i believe most people on this board would feel the sameway. Anyone that has any interest in programming....
One thing i learnt since i was a teenage training is its WAY more fun if you think on your feet each session and not follow some mindless numbers you have on a piece of paper - think 5/3/1. Your mental energy is better spent in the gym then looking for the perfect program online somewhere. Some people love set programs however. Whatever floats your boat.