I can't believe I'm even writing about ascending/descending AC. Blame Marcia, who's doing some fascinating work with the old Chainmail tables.
Look, here's my only take on ascending AC:
If you're using AC to reference a table, don't express AC as a number. You can use letters! Letter grade AC! It makes it so much less confusing when the number on the sheet is separate from the number you're rolling against.
So here we go: Unarmoured = D, Leather = C, Chain = B, Plate = A
If there's a shield, you get a +. So Plate + Shield = A+.
That's it. That's the take.