Here's what I've come up with (likely has a few kinks, seems a bit odd to look at, will fix up if needed later)
Magnitude Guide (Mostly based off of Solacis' Persona Scaling Chain Blog):
0 = Roughly comparable.
1 = Difficult, but decisive victory.
2 = Easy decisive victory.
3 = Effortless/Borderline stomp.
4 = Stomp.
5+ = One-shot.
?> = Higher, but otherwise unknown
Note that this scaling chain is assuming each character is at the peak of their strength for each key (so very casual stomps may be upgraded to one-shot's, for example). Also, someone will not get the "unknown" category if we have a rough idea of their strength, only if it's completely unknown or just a possibility.
Scaling Chain for God-Tiers:
SDT Dante = SDT Vergil = DMC5 Nero >>> Post-Fruit Urizen > Pre-Fruit Urizen >>>>> Early-Game DMC5 Dante ?> DMC4 Dante ?> DMC2 Dante >>>> Void Mundus = Sparda = DMC1 Dante (w/Sparda) > Mundus = Abigail = The Despair Embodied >> Baseline Low 2-C