Regarding question five, how did I arrive at that answer?
Well Boyle's law says that our initial pressure multiplied by our initial volume should equal our final pressure multiplied by our final volume. Now we knew three of those answers.
Initial pressure is normal atmospheric pressure, which tends to be about 1 bar- this varies a bit depending on your location, but for example we'll leave it that figure.
Our initial volume is 1500ml, adding both the ballast tank and the reservoir, so 1x1500= 1500.
Our final volume is 1000ml, so in order to have that reach the 1500 magic number, then we get a pressure figure of 1.5 bar. of course on a pressure gauge that would read 0.5 bar, because our gauge will be at atmospheric pressure like the rest of the kit.
Hope that makes sense.
All these questions were asked for a purpose. If you are building a WTC, then you really should know how much it will displace, and how much weight it will support.
If you are designing a ballast system, you might want to know how much extra to add on if you want to be able to trim your boat for different water conditions.
If the ballast system recirculates air, then you will want to know what pressure it will reach, to see if your pump can make pressure.
You can of course use solely empirical methods, but a few minutes spent with a pocket calculator can save hours and wasted material.