The secret to mounting the scope is to make sure the W & E are set dead balls on at "neutral" (or mechanical 'zero'). Turn either one of the knobs (say, Windage) all the way to one side. Now turn the knob all the way back, counting the MOA adjustment (a.k.a., "clicks"). Then divide that number of clicks by 2 (let's say your result is 120 clicks-- divide that by 2). Turn the W knob back 60 clicks. Theoretically, your W reticle is centered now. Now turn the Elevation knob all the way to one side. Turn the knob all the way back, counting the MOA adjustment (a.k.a., "clicks"). Then divide that number of clicks by 2 (let's say your result is 100 divided by 2). Turn the E knob back 50 clicks.
In my example above, W turned 120 clicks (30 MOA) from one side to the other, while E turned only 100 clicks (25 MOA)... this could happen if your scope came from the factory with the reticles misaligned--to fix the error, just run through this exercise 2 or 3 times; the more you do it, the closer to "neutral" you'll set your reticles.
When you've finished this, you can try and align your scope in the rings so that the "neutral" crosshairs are 'zeroed' on your target. This method will allow you the most W&E adjustments from your zeroed yardage.
"Free" mounting, like what's usually offered during a gun sale, won't often include this scope refinement. Sometimes folks will complain that they've run out of adjustment for either W or E--it's common to see someone complain, "my rifle's set for 100yd zero, but when I try to adjust the Elevation knob for 4MOA high (48 clicks), I run out of adjustment at 2 1/2 MOA (10 clicks)." It's because the scope wasn't bore-sighted with the reticles at a "neutral" center--instead of mounting a scope with neutrally-centered reticles that would look like this --> +, the reticles are actually oriented more like an upside down version of the Christian cross, (where the E reticle is biased low, in this example--kind of like an upside-down lower case "t"). If a scope is mounted in this manner, the E adjustment is half-used up during installation (and by virtue of the E's poor setting, the W adjustments are partially inhibited also).
**my MOA calculations are set on the generally-accepted value that 1MOA equals 4 clicks--however, there are a few scopes that have knobs that are set so that 8 clicks is 1MOA (and mebbe there are even scopes set at 6/1moa sad.gif ).