You did not mention which Extra 300 you have. Since this is the beginners forum I would guess you probably have the Park Flyers Extra 300 that advertizes, "Hover at 1/2 throttle"??? If it is I hope you have some experience prior to that plane. What will likely happen if you set your ailerons up the way you describe will be an adverse downward pitch of the nose. I know that does not make sence however, all but one of my models will do this when I apply Flapperons without mixxing it with up elevator. Since you do not have the ability to mix I would encourage you to learn to land it hot as opossed to adjusting the ailerons and elevator. Now I have no idea of your previous experience so forgive me if I presume to much.

Best of luck with whatever you decide.