RE: How to calculate ballast tank volume?
Well, I know next to nothing about RC subs right now, but since you haven't gotten a response yet, here's how I'd figure it out...
First, recognize that neutral buoyancy means the weight of your sub must equal the weight of the water displaced by your sub. This means if your sub displaces two gallons of water when totally submersed, then it will sink if it weighs more than two gallons of water, and it will float if it weighs less than two gallons of water.
So the hard part is figuring out the volume of water your sub will displace. If your entire hull is water proof, then you just need to caculate the volume of the hull. If you have WTCs inside and flood the hull, you need to calculate the WTC volume (based on exterior dimensions, not interior) and guesstimate the total volume that the hull material takes up (maybe you can neglect this, especially if the hull material itself has close to neutral buoyancy).
So my simple minded approach is to figure out how much weight/ballast you have to add to get you total sub weight > water displacement weight.
So lets define some variables and write some equations...
WS = dry weight of sub (empty ballast tank)
WWD = weight of water displaced by sub
WB = weight of ballast
so neutral buoyancy is when:
WS +WB = WWD
and you need to calculate:
WB = WWD - WS
You said that WS = 11+ pounds for your sub, so you need to calculate or guesstimate WDW and then you can find WB.
I believe 1cc of water weighs 1g, so 1L weighs 1kg (1L = 1 cubic decimeter = 1000cc). I don't know the English unit equivalents (the metric system keeps things simple, as you can see), but you can just take your measurements in centimeters, or look up conversion factors.
So if your sub weighs 12lbs, and it displaces 16lbs of water, you need a ballast tank that can hold > 4lbs of water.
Hope that helps... maybe one of the experienced sub guys here will chime in and provide an easier way to figure it out!