General rule of thumb is 10% (that's the default that ABRP uses). It would give you around a 15-25 mile margin, depending on various factors. Other than that, it depends:...how many miles of cushion should I add to the distance required to reach my next charge stop?
- Have you done any long distance highway trips before, and are you confident in your range estimates?
- What are the recent Plugshare reviews for the next charging stop? Is it reliable?
- Are you traveling during a busy time? Is the next charging stop likely to be available?
- If the next charging stop isn't available or reliable, how far is it to an alternative / back-up charging station?
- What's the weather? Will be cold? Will you be driving into a headwind?
- What's the terrain? Will you be driving up a mountain? Or driving down one?