This has become important to people using mass spec for biopharmaceutical QC with multi-attribute models (MAM):
https://www.tandfonline.com/doi/full/10.1080/19420862.2015.1069454
The new PeptideResult.ModifiedAreaProportion value gets at this for the set of peptides with the same unmodified sequence, but the paper above advocates tracking the area proportion of other groups like peptides with glycan modifications as %glycan = (area for specific glycan / area of all glycans) * 100, where they are actually talking about separate peptide sequences with specific glycan modifications.
Other discussions have included grouping peptides with missed cleavages, etc.
We should add a new Peptide property like Peptide.Area Grouping ID and an new proportion value like PeptideResults.GroupedAreaProportion based on %peptide = (area for peptide result / area for all group members) * 100.