What works for me is the most common advice, keeping it between 20-80%; charge more when you need the range. Charge to 100% and you get on the road 'pretty soon'. Ford says it's OK to charge to 90% regularly. I guess the ICE equivalent is people who get gas when the gauge moves much below FULL. I know about how much range+buffer I need and don't charge above 80% unless I'll need it.