You could, theoretically, define excessive profits by some mathematical equation that shows a ratio of expenses, revenue, and profits, and say "a certain percentage in this ratio qualifies".
Personally I think the real reason it is undefined is so that if perchance you get profit limits or higher bottom wages you can still press for more, and more, and more, and more........
You deliberately cannot reach your goal.
Actually, the real reason it remains undefined is because we're not talking about the economy on a microscale. At a macro level the economy has performed better when the labor share of GDP is 5 to 10 points higher than it is now.
It's about coming up with policies that help move the labor share back up to a more optimal level for strong economic growth.