A general tactical rule in sail racing is that if you pass three boat lengths downwind of an opponent you will be clear of his wind shadow.

I do find it fascinating when following someone while kayaking how long the vortices linger in the water from their paddle strokes.

If intending to use smoke as an obstruction, I would suggest that smoke move directly downwind 1/2 a card on the turn after firing, then dissipate the following turn.