Ballast tank volumes differ depending on the boat you're building, desired waterline and its method of construction.
You're approaching this problem from the wrong direction, first you need to build your boat, then size the tank using empirical methods. Trying to calculate it in advance is futile. Only a very simple shape can be calculated in advance and then that is assuming the hull has no free flooding area's.
How are you planning to make the hull? My advice to you would be to seek out a good secondhand fibreglass hull, or purchase one of the plastic kits I mentioned earlier, and concentrate on fitting that out with a dive module of your own construction.
Building a master and moulding it from scratch is long winded process, and not to be undertaken lightly. personally I would tackle that as a second project, as getting something going quicker will help maintain your enthusiasm.
Traplet is the best and cheapest place to get Norbert's book. The other sub publications are worth getting too.http://www.trapletshop.com/gb/p/11343/m ... rt-bruggen