Calculations will take you only so far, but the real work is practical observation and adjustment in water deep enough to accommodate the model submarine -- a test tank deep enough to permit the submerged submarine to sink and not hit the bottom of the test tank once completely under.
You can press the house tub into service, spend several hundred bucks for a galvanized steel animal trough, or you can go cheap and use one of those long and tall plastic dry-goods container outfitted with suitable strengthening elements to keep if from distorting one filled with water. That's what I did recently -- a lot easier than running up-stairs to use the bath-tub. Walmart has 'em for about twenty-bucks.
A quick trip to the back-yard #1 shed secured the wood needed to stiffen the plastic container (test tank) and reinforce the big card-table that supports everything. Without the reinforcement at the long sides of the flimsy plastic tank, it would bow out with the water pushing it out of shape, making the thing nearly useless. I also strengthened the underside of the card-table with a pair of two-by-fours to keep it from buckling under the weight of the filled test tank.
I had things done in half a day. Once filled with water I went about the task of checking out the five boats I'm bringing to the Groton Officer's Lake fun-run next week. And, in a few weeks after that, I'll use this test tank to check out six new boats I'm readying for the Newport News Fall Festival and NC City Lake all 1/96 'fleet run'. I'm old, fat, and don't want to schlep things between shop and up-stairs bath-tub as I adjust foam and fixed ballast weight as I trim out a boat. The new outside test tank is right outside the shop door. Perfect!