The bottle drill is a fairly overrated training exercise.
The most important aspect of a good stroke is "feel", and unfortunatly that is a hard thing to explain and pretty much impossible to teach. Part of it is most definately the timing that is explained above, stroking straight through the cueball explained above is critical, part of it is the "feel" of exactly how long the bridge should be on each distinct shot and that changes for each player, part of it is the correct stance and that again depends on each shot as shooting off the rail will be different then shooting a cueball 3 feet off the rail and a shot off the side rail on each side of the table changes yet again. With all the variety in this game it is virtually impossible to teach a person the "feel" of a good stroke.
Generally speaking the key is stroking straight through the cueball, the cue needs to come back straight and go straight through the cueball in line with the shot. If you do this correctly you will hit the ball exactly where you are aiming it, if it misses then at that point you know you simply aimed wrong, it was not the stroke at that point. Spin you play on the cueball will change things up abit and this is for sure not a thing you can measure and adapt for beyond shooting a ton of pool and getting a feel for how the shot plays. Heck, with a new cue with a different deflection when using siding this changes everything up and anyone has to relearn and adjust to the new cue's feel.
It is a very elaborate thing, long story short, shoot for 5 hours a day just potting balls, long shots, short shots, inside english, outside english, follow, backspin, mix it all up shooting everything, when you find a shot you have an issue with such as backcutting a long shot to the left with top left hand siding then focus on it and shoot it until you start to hit it in and feel how it is to stroke that ball in with that spin, ect... for 1 month straight and you will get more stroke. There is no magic way to do it, shoot a boatload of shots and hope you have the natural affinity that makes the improvement take place fast.