The bacteriostatic water with 0.9% BA is fine if the BA is the only other added ingredient along with sterile water. If it contains saline (sodium chloride or salt) then that could cause the peptide to precipitate out. If the solution is still clear, you're good.
If you mixed 6000 IU into 20 ml then each ml = 300 IU or one spray (0.1 ml = 30 IU) from the bottles I mentioned above. So if you want 30 IU in each nostril twice a day, that would be one spray each nostril, twice a day. With the spray bottles, it will be hard to be exact. The most important thing to know is how much one spray delivers (ml) because unless you know that, you can't determine how to mix the peptide (concentration).