in Code.gs [142:150]
function getDocIdOptions() {
var data_width = SpreadsheetApp.getActiveSheet().getDataRange().getWidth();
var options = [];
for(var i = 1; i <= data_width; i++) {
options.push({ value: String.fromCharCode(64+i)+":"+String.fromCharCode(64+i),
text: String.fromCharCode(64+i)});
}
return options;
}