I have a Magnum 91 4 stroke that is probably heavier than the 61. My battery is as far forward as the firewall permits. The receiver is just behind it. With this set up it was a perfect balance and did not need extra weight. Total weight of plane is just over 9 lb on a postal scale. Takes off at 3/4 throttle with a 13x6 prop. I set my elevator 1/2" up /down movement. This is actually too much once its air borne.

Check your float to wing incidence. Using the supplied mounts and hardware should give you about 2 degrees of negative (nose pointing down) I also have my c/g set at 75 mm from leading edge. I think the instruction states 75-80.

It takes off with a short run. I tried the flaperons and it did make a difference, but only used it once to see how it handles. It flies very gracefully and good flying characteristics. One warning, little more speed and altitude is your friend. I assume you have read the post regarding steep banking and spins.