The two lines are set to bracket 15-18" at 100yrds., they do not move. To bracket a chuck at say 400yrds. you turn the power ring and the magnification brackets the 15" chuck between the lines. Then look at your scale to see how many yrds. away it is, it should read 400, then if you have BDC-Bullet drop compensator you turn your elevation turrent to the right yardage.