I am with Brian425... I use the "precision data trolling app" to help calculate lead lengths with desired depth AND speed for a specific lure. It will calculate with or without snap weights. I also use the "Offshore tackle" snaps weights which are easy to use and I have never had one come off....
Let's say you are using a "Bandit deep diving lure and you want troll at 26' down at 2.0mph. The app tells you to let out a total lead of 84ft. So you then attach a snap weight 50' ahead of the Bandit lure and then let out an additional 34ft of line. Pretty easy and no guess work.