To make this work with indexing you have to get the current page number and page size.Once you have them you can always form the next RowNo.
var pageNum = $('.jtable-goto-page select option:selected').val();
var pageSize = $('.jtable-page-size-change select option:selected').val();
var RowNo= pagesize*pagenumber - pagesize;
Now replace the var RowNo= 0; with aforementioned.
But honestly no point calculating this from client side, rather get it directly from server side.