Given the English sentence
Call us when you leave so that we know that you're on your way.
I understand that it would be translated as such:
Llámanos cuando salgas para que sepamos que _____ en el camino.
My question is this: would the underscored portion ( _____ ) be estés or estás?
I would think that it would be estés because, as I understand it, the "rule of thumb" for subjunctive is that it is used in cases of doubt or uncertainty. As it is uncertain that tú will call, it is also uncertain that he will be on his way.
Am I correct in this understanding?