sighting at 30y and shooting at 15

"BayStYat"Does zeroing a scope at 30 yards effect your hold over/under at 15yards?
There are two common zero methods. In one the rifle is zeroed at only one distance. In the second the rifle is zeroed at a near and a far distance. In both cases the pellet rises after it leaves the barrel until it reaches the line of sight. If you adjust your zero so that the pellet has only one zero range, it will rise until it just touches the line of sight and then it will start to drop again below line of sight. If you adjust your zero so that the pellet has a near and a far zero range, the pellet will rise and then cross above the line of sight. It will continue rising until it reaches the distance which would have been your zero point if you zeroed with the first method. At that range the pellet will start falling again and will eventually cross the line of sight at the far zero range.

So with most setups if you zero at 30 yards your pellet will be below line of sight at 15 yards and rising.
 
It depends what you mean.

No matter what distance you zero your scope at, the pellet will follow a trajectory that is based on the speed, weight and pellet design. No matter what, the poi will only be in the center of the crosshairs at one or two points in that trajectory which means that, for most shots, there will be some corrections to do if you want to aim precisely.

It's a good idea to punch your info into one of the ballistics calculators so you can take a look at the trajectory of your pellet. 

If you have an SFP scope then you will want to find out at which power setting the mil dot reticle is correct and if you are not using that setting, then you will need to work out the adjustment for the one you are. 

 
As zebra points out, I'd also recommend playing around with Chairgun or Streloc to figure out your best zero and trajectories (hold over/under) for the ranges you anticipate shooting. You'll need a chronograph or a pretty accurate idea of your muzzle velocity, as well as an accurate measure of the centerline of your scope over the centerline of your bore.

Depending on where you sight in and the trajectory, you can get POA/POI at 2 different yardages as displayed below (sighted in at 40yrds, also POA/POI at ~20yrds) just as oldspook describes. I chose this zero as most of my pesting is 20-40 yards.