Let's say you have Thac0 13 and you roll a 5, then you hit AC 13-5 = 8.
Let's say you have +7 to hit and you roll a 5, then you hit AC 7+5 = 12 (the equivalent of AC 8 with Thac0).
In other words, the difference between Thac0 and to-hit is just that you do a subtraction in one system, and addition in the other. When they changed from Thac0 to to-hit, I really didn't really get the benefit, as I know both subtraction and addition.
Thanks for the implication contrary to the evidence I am capable of simple math but the target 20 method just clicked for me in a way that thaco never did. I can do thaco but I need to think about, with target 20 it just happens.