I just started adding sambhavi to my SBP. Before that I would, as Yogani suggests, do a moment of sambhavi when my breath was at the top and my visualization was located at my ajna. So when my eyes were looking upwards, the visual location was on the ajna. I was physically and mentally looking at my ajna.
When I starting adding sambhavi, I was looking at my ajna and still visualizing the silver tube going up and down my spine but I was splitting my attention.
I tried doing sambhavi and basically locking the muscles and visualizing my normal up and down the spine. It seemed to work better but it felt like I was looking upwards with my eyes but mentally not looking at the ajna but looking at my spine.I hope this makes sense.
So physically my brow is gently squeezed and eyes looking between the brow but mentally I'm visualizing the silver tube going up and down my spine. My eyes are looking upwards but looking at the spine (except when I reach the ajna at end of breath).
Sounds like you are on the right track. See this AYP Plus lesson for confirmation and some pointers on coordinating sambhavi and spinal breathing pranayama: http://www.aypsite.com/plus/131.html