RX, The bottle pressure will decrease as you spray, no matter what you do unless you back pressure it with nitrogen. That happens because the bottle gets cold when the pressure is released, and colder is less pressure. If you start you bottle cold, the pressure will not change much until the N20 starts to run out. You should be able to get at least 3 lbs in your bottle if you keep it cool. How many 15 jets are you planning to run? Run a lb of N20 through your system and time it. Using the correct mixture 2 lbs per minute should be about 20 hp. Run it a tad rich.
You can search the site for N20 and look at suggestions. I recommend before you spray anything in your engine you figure out what you are putting in there. Without flowing your jets you will not know what you are putting in your engine.
We ran ERC 110k as motor and enrichment fuel with no problem.
Good luck with your project