I reckon drying marks form if the PhotoFlo rinse dries before it has had time to run off the film. This is what I do to 100% avoid drying marks:

Hang the film while it is very wet and streaming with PhotoFlo solution BUT hang it edge down at a 45 degree angle. The liquid only has to flow across the film to the bottom edge and then along this edge to the bottom clip. This takes only a few seconds, not the minutes it would take for a bubble to drift vertically down from the top frame to the last frame. Actually I do occasionally get faint drying marks but they are all on the edge of the film and never on the picture area.