https://en.wikipedia.org/wiki/Aspartame:
“Aspartame is an artificial non-saccharide sweetener 200 times sweeter than sucrose”⇒ I expect they would like to see something like “39 REAL”, respectively “40 ARTIFICIAL” on the label.
Of course there’s a can of worms in that there are multiple ‘real’ sugars (https://en.wikipedia.org/wiki/Sugar) and producers will try to talk down that 200, but I think that’s solvable.
What’s less solvable is that producers will talk down the unit size used to compute the amount of sugar, just as they do with the unit size used to compute the number of calories.
Alternatively, this could be shown as a sweetness rating with “x% artificial” added, or just as a sweetness rating alongside the existing ‘calories’ rating.