^That just means that whatever drug came in the bottle was dissolved at a concentration of 50mcg per spray, it doesn't help you figure out how much ethylphenidate will be in each spray if you fill the bottle with an ethylphenidate/water solution. But knowing the number of sprays in a bottle is helpful. Does it say how many ml the bottle holds? If not, what you would need to do is fill up the bottle with water, pour it
all out into a measuring device (something with very precise markings for measuring small quantities of liquid, like maybe an oral syringe, etc) and measure how many ml of water that was.
Please keep in mind that you can't be sure
exactly how much is going to be in each spray, as the sprays may not be all
identical in volume, so you want to err on the side of caution and have not too much ethylphenidate in each spray. However, you also don't want to have your solution so dilute that you need like 10 sprays for an average dose, as it might be hard to keep that all in your nose without it dripping out.
Once you know how many ml the bottle holds, then you can do the following:
1) Figure out how many mg would be in each spray if you dissolved the full gram (1000mg) of ethylphenidate into the full number of ml that will fit in the bottle. So 1000mg divided by 150 sprays = 6.66mg per spray. If this number was fine with you then you would just dissolve the 1000mg in the full amount of water that the bottle could hold. If you wanted to do a dose of around 40mg you would use 6 sprays. However, it would be a lot easier to calculate your doses if you put a number like 2mg, 5mg, 10mg, or 20mg in each spray, as opposed to 6.66mg. To do that, instead of dissolving your ethylphenidate at this point you would do the following:
2) Judge approximately how many sprays would equal 1ml. Here's an example for if your bottle held, say, 50ml: 150 sprays divided by 50ml = 3, so then you would know there would be around 3 sprays in each ml.
3) Decide how many mg you want to be in each spray. If you wanted about 10mg per spray, then using the example that your bottle held 50ml, then 10mg x 3 sprays = 30mg per ml. 1000mg divided by 30mg = 33.3, so you'd dissolve the 1000mg in 33.3ml of water. So in that scenario, with a 50ml bottle, you would only be filling the bottle only part way full. If you wanted to do a dose of around 40mg, then you would use 4 sprays.
Please feel free to let me know if you need any help with anything

Hopefully the amount of ethylphenidate will fully dissolve in the amount of water you use.