I shot at decoying geese, first shot 20-30 yards. Used a factory mod choke.
The lead depends on what load you are shooting, wind, way the birds are flying, etc.. its just something you have to learn. Practice practice practice, more time in the field (or water) the better hunter you will become.
I bought the kent impact tungsten matrix from gander mtn I believe, it was around $30 for 10 shells.
Its not something you want if you are going to pass shoot and just throw shot at them, and you probably wont want a 20 gauge for that anyways. For decoying geese you dont need much, a 20 gauge will do the job.
And before anyone says anything, I was shooting 13-15lb giants.. the #3's did the job just fine. Tungsten has much more knockdown power than steel, a #3 tungsten is like shooting a #1 steel.