Honestly that sounds even better than what I proposed. Maybe put the button right below the "Random HP" button on the Levels sheet and label it "Backfill Levels". Add a tooltip that says "Fill blank levels starting from the highest populated level and fill all lower un-populated level entries to duplicate selected class(es) and favored class bonus(es)."
Add a "Cost Override" column to more items in the Equipment sheet
Add a "fill levels" button to the Levels sheet
Goblin racial favored class bonus does not appear in dropdown for Rogue or UC Rogue
Unchained Barbarian is unable to select "Improved Damage Reduction" rage power more than once