You lookup the value of the data with findData()
and then use setCurrentIndex()
QComboBox* combo = new QComboBox;
combo->addItem("100",100.0); // 2nd parameter can be any Qt type
combo->addItem .....
float value=100.0;
int index = combo->findData(value);
if ( index != -1 ) { // -1 for not found
combo->setCurrentIndex(index);
}