I used to get the same issue when I was starting to learn 270 offs.
3 main things you gotta remember, if you can't think of it all at once, do the rail and 270off doing 1, then 2, then get all 3 together and your set.
1) pop off. practice just popping and land toward whichever way you wanna learn to 270 off, i started popping to landing switch since i was trying to get blindside.
2) wait till the very end of the rail, everytime i hit the rail it was cause i popped way too soon
3) start your rotation a bit on the rail, essentially the second you actually get off the rail your tips should be 180* away from your 270, unless your doing a pretty high rail