Re: newbie question
Adjust your scope's zoom. If your reticle gets smaller at lower power, you can range at any distance/power. If it does not shrink or grow, the manufacturer will probably suggest 10x/max for proper ranging.
You should also test it at 100M, 200M, 300M etc to verify it's accuracy and movement. Be sure to work it in before using it. When you mount it, there is also a way to insure you have it mechanically zeroed in the rings before placing the top ring on and tightening it.