That's a tough one. I would say that you can't really articulate the subtle flexing of the pulse, it has to be felt, not counted.
One of my original teachers (committing the 'ultimate crime' of moving off 'out of phrase') would suggest counting (from a man's LF walk into a feather step) "Slow, Slow, Not-so-slow", and I have always found the idea helpful. In the first steps of either natural or reverse turns, I usually try and hasten (only very slightly) the first step, so that I have more time to swing well forward and around the lady, so that I can give some time back, making it easier for lady to complete her heel turn and rise, ready to move on.
Foxtrot pulse has to breathe, partly to give the feel of continuous body flight across the floor, but also to facilitate the mechanics of the turning figures and the footwork requirements.
I fell out with a teacher, many years ago, because he insisted that length of each step should be more-or-less the same, regardless of the slow or quick timing, but with more-or-less equal body flight too. This, of course, is impossible. If I want to maintain a uniform rate of movement across the floor, while halving or doubling the step speed, I must vary the step length, or alternatively, if I keep the step length constant, while having or doubling the step rate, then my rate of body flight will constantly change.
What he wouldn't concede, is that the two are not irreconcilable once you recognise that the position of the body over the standing foot can be altered considerably, to allow the latitude to create what is, really, an illusion.
A very good foxtrot dancer moves with considerable latitude regarding the basic pulse (without ever loosing connection with it, particularly for heel leads). The casual observer would say that all the steps were nearly the same - where were the slows and quicks? It's not an easy thing to provide any formula for, though, that another dancer can reproduce. A good demonstration (or careful observation of experienced dancers) is what is required, here.
