You asked for advice?
First thing I'd do would be to
mark the desired waterline using a soft pencil at the bow, stern, port and starboard.
Float the hull in the kitchen sink and add blobs of plasticine wherever it's required to get it floating at the
correct depth.
Using the very same soft pencil, push the hull gently sideways at various points along the gunwale until you can get it moving sideways without turning. Explain to the wife or significant other that you're
not playing about, but conducting a series of
essential hydrodynamic tests.
This point on the gunwale is the hull's
centre of lateral resistance. You don't need to be too scientific here, as this point may well move fore or aft a little when the hull's heeled.
Take out the boat, and balance it on its keel on the now-dripping-wet soft pencil. This point is the hull's
centre of gravity when loaded.
Splitting the difference between the CLR and COG, along with squinting a bit, should give you a good guide to where the fin is best placed. Mark this location with the
back-up soft pencil..
Now scoop out the plasticine. The mass of the plasticine is the displacement of this hull. If this mass is less than the mass of the radio gear, things are looking good. The difference between the displacement and the radio gear is, of course, "everything else". Being a small model, I'd be aiming for maybe half of this difference (assuming there is one!) being used for a ballast bulb.
I have a question about the chosen rig: gaffers are, undeniably, the second best looking rigs for boats, but it makes for a lot of mass up top. Bermudan rigs are dull, gunter little better, but for a hull like this I'd be sorely tempted to go down the (jibless) lugsail route. A lowish, squarish, inefficientish sail that would suit the hull and not overpower it.
Best wishes with a fascinating project!
Andy