Well I've never taken an up close look at a nasal spray bottle so I'm not exactly sure how it works. For this purpose I'll assume that you can easily take it apart, dump out the liquid and refill it with whatever.
Ketamine is very soluble in water, something like 20g/100mL so first off, fill your nasal spray bottle with water. Spray one shot into a small container like a shot glass and measure how much water comes out. Do this a few times to make sure it's fairly consistent. Then dissolve a known amount of K in a known amount of water, for experiments sake let's say, 1g dissolved in 100mL of water. Pour it into your nasal spray bottle and voila. You now know how much K you will be getting with each shot. If you measured 1mL of water coming out each time, then you will be getting 10mg per shot. If you measure 2mL of water coming out each time then you will be getting 20mg per shot etc.
If this will actually work, I have no idea but theoretically it should.