I agree - the bathtub test is the only way to do it - whether you use a bath or swimming pool doesn't matter - the test is the same.
You should previously have some idea of where you want the boat's waterline to end up at. When you put the boat in the water, with the battery and all heavy stuff installed, see where the boat actually sits. Then apply ballast to get it down to where you want it to be.
MTM has it right - the location for the ballast is just as important as how much. It should be located as low down in the hull as you can get it, for stability. Placement fore and aft is a little more fiddly, as the boat probably will need more ballast at one end than the other; placing the ballast near the centre of buoyancy will tend to bring the hull down more or less evenly, while placing it near the pivot point will make the boat more "lively" when steering (not necessarily an entirely good thing).
Sheet lead is often used for ballast, simply glued in place where you need it. Lead shot is also good, as it can be poured into place, brushed around until you are satisfied, then fixed in place with a good dose of spray glue.
Des.