If you have a LR with 18" aero wheels and flat ground, you've got plenty of range. Keep the speed under 80, make sure the tires are inflated, charge to 100% before you leave (and be careful, because you won't have any regen) and you'll be golden. If you have an SR, SR+, or MR, or have an M3P, you'll be cutting it a lot closer.
Use Navigation to plot a route to your destination, then pull up the Energy graph, and select the Trip tab. The car will show you what it thinks the battery state of charge will be all the way to your destination - as long as it estimates you'll get there with more than 10%, you're in pretty good shape. Watch this graph on your trip - it'll show an updated state of charge graph; if the estimated charge at your destination is falling, slow down. If it's rising, you can speed up. It's pretty simple - the most efficient speed for a Model 3 is somewhere between 30-35 mph IIRC, and it gets worse every mph faster you go. You should get rated range at around 65 mph in perfect conditions - flat ground, warm temps (80), no wind, no rain. Add in any of these, and the expected range will drop.