Is there a chance that you are straining to hard with sambhavi mudra? Part of sambhavi is to slightly furrow the brow and this can cause a bit of tension in the forehead if taken too far.
The third eye/ajna is in-between the eyebrows and slightly above them on the forehead....not in-between the eyes.
I'd suggest laying off of sambhavi (or "loosening" it a bit) if you are using it, and if not, perhaps take a small break from SBP for a while if you are finding things too uncomfortable. Just a suggestion.
You are doing the practice correctly. The ajna chakra is located in the centre of the forehead. If you are experiencing tension, and that is uncomfortable, then cut back on the practice a bit, say to 5 minutes twice a day instead of 10. A certain amount of tension is normal at a certain stage in the practice of spinal breathing pranayama as the sushumna nadi becomes activated and begins to fill with prana.
Hi - I agree with Christi. I have experienced ajna to be anywhere from the top of my forehead down to my upper lip - with the most activity being directly between the eyebrows. And so the target point for the top of SBP is very difficult to miss.
Thanks for the advice. I think I might be straining sambhavi a bit too. You really know SPB and other energy practices well. No wonder your sushumna is "This Big!"