When travelling at motorway speeds I am lucky to achieve 2.5 - 3.0 miles per kWh so plan accordingly and also add in a a 50 mile contingency in case the weather requires use of extra systems like demist functions or heaters. The contingency is also in case your planned charging stop has faulty chargers or excessive queues and you need to find an alternative.
I find the best plan for charging stops is little and often, so even on a trip where I could probably get to my destination with a single stop it will usually be faster to plan in 2 stops that will take less overall time (remember the car will charger faster between 10% and 50%).