pivot foot: drag vs lift

[rsl]

The definition of travel says

"The pivot foot may be lifted, but not returned to the floor..."

When a dribbler stops too fast, it is often the case that he will drag the pivot
some distance without actually lifting it from the floor. I call this a travel.

It also happens that players lift the heel of the pivot foot and pivot on the toes. If they shift their body too much, it is easy in this case that they drag the pivot foot with the toes touching the floor. I usually call this a travel.

When, under pressure, a player spins back and forth rapidly on the pivot foot, the pivot foot usually moves slightly on the floor. I rarely call this a travel.

How much does the pivot foot have to move, without lifting, to be a travel?