I think the easiest way to tie a parachute post is with any synthetic material. I say this because you can tie it in long and then trim it. all the natural materials i know of for this purpose need to be tied in at the given height. I generally use polypro yarn and trim it as the last step when tying the fly.
I must admit however that i don't tie parachutes much because i view them as more hassle than they are worth. I'll use a no-hackle in the same situation, as they are much easier to tie.