At first I was like you thinking that keeping the internal battery at 80% would be optimum for the battery, but as it turned out it degraded my battery. My internal battery after 1+ years depletes rapidly (when not attached to the battery mod). And there is MUCH discussion here on the forums with others who have found the same thing.
I'd recommend using the battery mod like you plan to, but set it to off (new mod update setting) until you get down to 50 or 40% and then turn it on and let the battery mod do it's thing taking it back to 80% with efficiency mode. I think that way you will extend the life of your internal.
It's a shame they haven't implemented a way to do that automatically with the battery mod software, but hopefully in the future they will let you set the actual percentage you want the battery mod to kick in in addition to the 2 modes they currently have.