The "packing" is filling the space inside the packing nut, around the
valve stem & above the washer that goes against the valve body.
Try to visualize the amountof space & put it that amount of string. I
using use "pre-formed" packing not string. If I do use "string" I use
the teflon type. I using try to put too much in & the nut won't engage
the thread; I cut off a little & try again. That way it's one
I also "wind" it such that tightening the nut continues to wind it,
not unwind it.