You can't set a 2015 Leaf for 80% charge, I'm afraid. That ended in 2014. You have to just estimate when it will reach that point, and unplug it then.
The best strategy would probably be to charge to about 70% for the short trips, doing it every other day if that works out ok, and then charge to 100% right before the longer trips, letting the pack equalize but not letting it sit long at 100% indicated charge.
Scarlet Ember 2018 Leaf SL W/ Pro Pilot
2009 Vectrix VX-1 W/18 Leaf modules, & 3 EZIP E-bicycles.
PLEASE don't PM me with Leaf questions. Just post in the topic that seems most appropriate.