When it comes to reading the lines on a crossbow scope, the first thing you should know is that in most scopes, the first line is usually 20 yards distance. For such scope, the next line will be 30 yards, because scopes increase at a range of 10yards. Therefore, the formula is Line1 =20 yards, line 2= 30 yards, line 3 = 40 yards, and so on. However, some scopes are calibrated differently following a certain speed. For a bow that shoots at a range of 350 fps, 30 yards would be 27 yards. This is majorly because of speed frequency.