I say 30% is just the way it is supposed to be used for most of the people. The 3D effect comes from the two separate images, it's not like you get MORE 3D if you push it all the way (even if the depth field looks deeper)...
Also Retro Link, if you play a mission correctly you should be aligned to the landing path / pad. Practice at lower level, usually you get the grip on speed and precision for landing there. (And there is no wind in this game unlike Pilotwings Snes, only maybe shifting pads in two jetpack missions)