Keep in mind, the 20% number was just an example- there should be a tipping point where players would have to make a meaningful choice.
Maybe losses on auto if you chose the right units would be 1,000 ± 200 but a skilled manual player could do it with 300.
I think it might be a cool idea if coupled with a battle preview.
You look at the set up, set your troops, and the game could even tell you that you will lose 1,000 ± 300 on auto, then you decide "Hmmm with that terrain, I think I could do much better" or "Nope, that set up is hard, and I'm a lazy soggyshorts, I'll let my generals handle this fight."