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Because deep down inside i don't think you're an alias, I'm going to give you some advice.
My buddy throws them once in a while. I mean as far as jump size goes, i'd say at least 50 ft for sure, but make sure it has a lot of kick. The initial kick will help you bring it around (but you should probably know that from regular backs already). I mean when it comes right down to it, you kinda just have to huck it out. It's not a trick where you just ease into it. Honostly, maybe wait until summer to try on a water ramp or something. It's a fairly dangerous trick. At least thats how my buddy learned them.
I would most deff practice into at least a pool and obviously on a tramp. Like the guy above said you cant work your way up to double backs you just gotta giv 'er. I think GotamaBC can do them, maybe pm him for some pointers.