There are plenty of legs across the country for which Nav/Trip Planner will leave you stranded on the side of the road at worst, and driving 20-30mph below the speed limit at best. There are also plenty of flat or downhill stretches with little wind and good wx for which Nav/Trip Planner does a marvelous job.
For unfamiliar stretches, do keep evtripplanner.com and in particular the downloadable .csv file in mind - especially for an understanding of which legs have significant net positive elevation.
130-140% over rated is a good rule of thumb, as is, for challenging legs, just adding 30% on top of whenever Nav/Trip Planner says yer good to go - which tends to be at 7%-13% over rated. Great buffer for flat warm non-windy ground. But if you want to avoid an almost immediate and arbitrary route reversal back to the last SC for half your leg (which itself can lead to unpleasant and unexpected results) and/or constant warnings to drive 45mph, do realize that it is more efficient to charge to, say 20% over rated at minimum, which might take an extra 12-15 minutes, than to lose 20 minutes or more driving below the speed limit and sometimes by a lot. What a waste of a perfectly good 80mph speed limit in many cases.
Over the course of a travel day, those 20 minuteses can and do add up. I like adequate amounts of sleep. Nav/Trip Planner negatively impacts that unless I make adjustments. Fortunately, those adjustments are easy to make.
Another example would be Nav/Trip Planner's penchant for avoiding interim SCs if it perceives you have enough charge. Well, again, what it perceives and what is realistic are often two different things. Note after tapping the Trip button the "Remove charging stops" option. Select that once in awhile for an idea of what other charging options exist. If it's windy and uphill, an interim stop is often not a terrible idea.
And so it goes. All it takes for a bad experience on the side of the road is to blindly follow Nav/Trip Planner and to have it be wrong once. It's wrong a lot, actually. Usually that just costs some extra time (see slowing down to get there above) - no big deal and lesson learned. However, there are some times when you'll be SOL, and a good way to avoid that is to simply charge a little extra until you've had the opportunity to transit the leg, ideally in both directions, at least once.
Once you know, you know. Easy peasy.