At your size3, I think you need about a 58-65 cm fin in your Rio.
Big guys need big fins.
As far as how much volume you "really need" I don't have a formula for that, but I'm pretty sure with your skill level now, that's going to be somewhere around 130 liters or more for a 240 lb. guy.
As you gain more skills, you will be able to get away with more of a "sinker" board, but
for now I'd look for 140 liters at least just to give yourself some reserve.
If you already have the mast foot 1" from the back of the slot on your Rio, and you are in the farthest back and outboard footstraps, you don't need to move anything back further.
Try the larger fin, I think that will make a tremendous difference.
Hope this helps,