I just purchased a Nikon Monarch 4-16x42 with BDC for my .243 WSSM. I was reading the direction and am a little confused about how this scope functions. Basically, do the zoom power effect the BDC. For instance, if I find that the 3rd circle is my 400 yard hold point and I use 16 power to sight in. Do I always have to shot 400 yards with it on 16 power. If I turn the scope back to 10 or 12 will can I still use the same hold point for 400 yards?