A 2.4GHz radio will not work for submerged operations. At best you might get an inch or so of depth before loss of control. Water just absorbs the radiation energy. (Think what happens when you put a cup of water in a microwave. It uses the same frequency.
By itself, weighing your hull will *not* yield a very useful data point for sizing a ballast tank. If you do not want to do it by trial-and-error (which most of us do!), then what you need to know is what is the displacement
of everything that sits at and above the surfaced waterline. That will tell you the volume that the ballast tank needs to shift/move to allow the sub to settle to a submerged state.
Using a 75MHz radio is the correct choice if you are in the U.S.....if you are not, let us know and we can tell you what frequency you should use.
-hope this helps,