excelExport: function (e) {
          let gridHeight = $("#grid").height();
          if (gridHeight < 700) {
            $("div.k-loader-container-inner").addClass("smallHeight");
            
          } else {
            $("div.k-loader-container-inner").addClass("biggerHeight");
          }
        }
...
<style>
        div.loaderPosition {
          position: absolute;
          top: 50%;
          left: 50%;
          transform: translate(-50%, -50%);
        }
      div.biggerHeight {
        position: absolute;
          top: 50vh;
          left: 50%;
          transform: translate(-50%, -50%);
      }
    </style>