I didnt read the other ones because im tired, but here is the comprehensive guide to 270 ons
Do not line up on an angle, it might center you on a short box but anything longer and your fucked.
come into the rail super forward, set a big lofty 180.
[b] the reason you are landing on the sides is because your setting the rotation early and rotating off the lip, be patient [/b]
Unlike jumping, spot down between your legs at 180 so you know when to drop your skis that extra 90. Then keep your arms at your sides and ride out sickly.
So, why are you only setting a 180? setting a 180 then dropping your feet doesnt make you carry all the momentum setting a 270 would. Thats why kids always over rotate and go ass over balls. Try it on the ground at home or something, do a 180 but put your feet down a 270, then you will be like ohh shit i see now thanks for the hot tip.