How to Set Loader In Center

Question

 How to Set loader In Center 

given Bellow In css i am using 

.se-pre-con {

position: fixed;

left: 0px;

top: 0px;

width: 100%;

height: 95%;

z-index: 0;

background: url('/Images/loading2.gif') center no-repeat gray;

background-size: 100px 100px;

}