Right Hand Motin -- Left Hand Synced?

I have a question that more experienced\pro players may have considered or worked there way though at one time. 

When striking a string with the right hand fingernail, the striking finger nail pushes through the string it has struck and comes to rest, briefly, on the string below it. That striking finger nail at some point begins a recovery movement, upwards, to some maximum upward point before it then begins its next down stroke toward its string strike for the next note. 

So my question is, if you were to be executing, say a hammer on following the fingernail strike, is there some exact point in the recovery motion of the right hand that the left hand strike for the hammer on is synced with?

Is the hammer-on sounded with the left hand before the right hand fingernail leaves its brief resting position?

or 

Is the hammer -on strike sounded with the left hand when the right hand recover motion is exactly at the top of the recovery stroke?

or 

does it matter?

I have noticed, for instance, that the second note eighth note of a quarter note pair - the on of a hammer-on, the off of a pull-off are slightly rushed in my playing. They were close, but not really crisp. The thumb was solidly on time.

I detected it as I was trying to increase the precision of each note in my playing. In doing so, I took some 4\4 tunes and played them very slowly. I played with the metronome and, for instance, if a measure had 4 quarter note pairs in it,  each note in the measure was played on a click of the metronome. It was then that I began to hear the rushing of the second eighth note as it didn't land on a click. I also noticed that on a hammer-on vs a pull-off the right hand was in a slightly different portion of the recover stroke. It was then that I began to play around with this concept of a sync point.

I am probably over analyzing this, but it seemed that there should be a common sync point for the right motion with the left hands movements when performing the slide, hammer-on and pull-off? 

I am probably clear as mud on my question, but. I'd appreciate input...

Thanks..

Mike