My swing is a little rusty but if you add an Object in a comboBox, the toString() method will be displayed. And when you try to get the selected item it will return the entire object.

So have an object Person with attributes: name, percentage.
The Person should have a public String toString() { return name; } method.
So when you add the Person john with values:
name="John"
percentage=0.15
to the JComboBox it will be display what the toString method returns: "John"
When you do combo.getSelectedItem(); NOT combo.getSelectedIndex(); you will get the object Person:
Person selectedPerson = (Person)nameCombo.getSelectedIndex();
then selectedPerson will not only have the name but the percentage as well. So you will get it:
selectedPerson.getPercentage(); and multiply with whatever you want and set it to another field.
I have never tried the above approach, but I think that it will work. Also I hope I understood correctly you problem.