Flat numbers also make things easier to add effects onto. Like if you need to make a Tricky roll against Breath and make a Hard roll instead, where you'd normally take half damage you take no damage. Same with poison saves and getting a Tricky result. Rolling a 20 on a Red save would give a similar bonus effect.
There are some notes on the digram to show how to transition characters to this method more easily, and specific notations with my cowboy dnd game in mind. Maybe more annotations as we go, maybe fewer, but otherwise I think it's got legs.