Fill a bowl, with some water. Drop a fly into the water, as if it was landing from a cast (you might want to attach a bit of tippet material, to the eye of the fly).
If the fly falls over, as you have stated, then cut material from the post, until the fly lands properly. Then you will know, how tall the post should be for that pattern in a given size.
The fly tipping over on the water, is most likely caused by, too small of a diameter base (parachute hackle), in relation to the length and amount of material used in the post construction.
[This message has been edited by Steven H. McGarthwaite (edited 22 March 2005).]