I hear ya--my thought is maybe keep it around 70 for your outbound leg and see how your range holds ups. Without the energy app, we are all flying a little blind, which kinda sucks. You can then make adjustments on speed on the way back and for subsequent trips. Alos a good time to load the PlugShare app on your phone. At those speeds, wind resistance increases as a square (approx) of speed, so a 10 mph difference in speed can make a big difference either way.
My own story about this: my office is 220 miles round trip to the Bay Area and my P85 had a range of 265 miles, so I had 40 miles to play with (this was 2013 so there zero charging infra on the route). My first trip, I was so paranoid I think I drove 60 the entire way down. My wife thought EVs were a science project back then and if I had to be flat-bedded home, I would *never* hear the end of it. Luckily, the car did fine and I was able to drive 65 on the way back and rolled into the garage with 20 miles of range. Luckily the Fremont Supercharge opened a couple of months later.