Based on advice from some spey fishers on the International Speycasting website, I ordered some poly leaders for my 7wt from Walt Winter at ezflyfish.com and some for my 10 wt from Frank at LoopUSA. Walt's are 12' (including your tippet of 3 feet) and Frank's are 15' (I don't know if that includes the tippet since they haven't arrived yet, but I assume that it does.)
There are some recipes in the archives of the International Speycasting website for leaders, but I found them unsatisfactory because they didn't start with a leader diameter that reflected a percentage of the line tip diameter, nor were they long enough. I have read that the leader should be of the same length as the rod using it.
Now, having said all of that keep in mind that I am a long time flyfisher but a neophyte when it comes to spey rods. Anything I say should be checked out with those more knowlegable.