for example a .223 with a 45 grain lead will be dead on around 40 yards this is first zero and dead again around 200 yards being the second zero.
This depends greatlyon how high the crosshairs are above the bore and to a lesser extent on the ballistic co-efficient of the bullet used and the velocity delivered by the load..With some mounts and with some loads it will be very close,but with others,it may not be close at all.