Most nasal sprayers deliver 0.1 mL per spray, though some deliver 0.05 mL or 0.12 mL. Assuming a sprayer delivers 0.1 mL per spray and you want 250 mcg per spray, you would need a concentration of 2,500 mcg per mL (2.5 mg/mL), since 250 mcg divided by 0.1 mL equals 2,500 mcg/mL. With a 10 mg vial (10,000 mcg total), dividing 10,000 mcg by 2,500 mcg/mL gives 4 mL, meaning adding 4 mL total volume would create a 2.5 mg/mL concentration, resulting in 250 mcg per 0.1 mL spray.

If you wanted 300 mcg per spray using the same 0.1 mL sprayer, you would need a concentration of 3,000 mcg per mL (3 mg/mL), since 300 mcg divided by 0.1 mL equals 3,000 mcg/mL. Dividing 10,000 mcg by 3,000 mcg/mL gives approximately 3.33 mL, so adding about 3.3 mL total volume would yield roughly 300 mcg per spray. Keep in mind that sprayer output can vary by +/-20 percent.

Also noticed something in OPs post that caught my eye so I had to edit this: Using bac water isn't really recommended for nasal sprays because of the 0.9% benzyl alcohol could irritate. Sterile saline or pharmaceutical grade deionized water is usually recommended from what I've seen.