I'm running a 15-4 on my little ultimate with a OS 91 Surpass, your magnum should turn a 15-6 I would think, although I'm not a Magnum expert. Try to alter your approach by cutting the throttle sooner on downwind and make a level turn to final at about 25 feet and give the throttle 3or4 clicks of power and fly it with just enough elevator to keep level, then use power to control descent, I know this is easy to say on paper, but with enough practice, this would be the prefered way to land. The senior being a floater might require that you extend your downwind out a little longer, which will force you to fly to the runway. It's always easy to add power coming in if your to slow. If you tend to wait to cut power until your final turn, that's usually to late.