In terms of the length of tippet to add, keep in mind that you want your sinking Polyleader to pull the fly down in the water column. If you use too long a section of tippet then the Polyleader goes down and the fly stays up. Not good.
In general, 3 - 4 feet of tippet is ideal. It is long enough to separate the fly from the Polyleader but short enough to get the fly down.