I too use that program, and Strelok as well. I was getting variances, until I rechecked my scope height, and then everything came into play as it was supposed to! Changes in velocity due to temp changes will definitely change things as well. Maybe try a temp -velocity table, and then use that data when conditions change. Both of those programs are totally dependent on Accurate input. Any variances, effects output, and outcome. I like the 4dof the best! Good luck.The bc is automatically set when you pick the bullet from the list in the app. I figured since it's hornadys bullet and there app it should be spot on.