The center of balance for your particular boat IS where it IS. You put the wheels under the boat until you find the perfect balance point. Nobody can answer that question for you.

For a Hobie 18 the balance point was at the shrouds. For the Hobie Wave it is well back from the shrouds, between the two molded-in seats.

If you think you know the perfect balance point for your boat and the back end is still too heavy, maybe it is full of water.