Yeah, but lets look at what
actually happens (using ACs from actual monsters in the books (over 800 of them, FWIW):
View attachment 262668
So, I've done an extensive analysis of this and that is why I said you don't need a 20 by level 8; you
barely need an 18 really. Honestly, you can even "get away with" just having a 14 to start, but you'll feel the pinch if you don't bump it to 16 at 4th level...
Anyway, this is why I can just take half-feats at 4th and 8th for +1 bump to modifier, and again at 12th and 16th for the last +1 bump. I've
NEVER, EVER spent an ASI on a +2 to a single ability score and the math works just fine without it.