5% = 50 miles would be 10 miles per %? Wouldn't that give 1000 miles on a full charge?
Well, you have 4.5% of your battery (maybe, not recommended to find out how well the BMS is estimating, though!) available below 0%, which you never want to use. So that 5% would be 3.5kWh+3.7kWh = 7.2kWh
So you'd have do do 144Wh/mi, which would definitely be worth freaking out about. Maybe if you drove 20mph you could make it (two and a half stressful hours!), but probably not.
I assume he was just proposing a scenario where freaking out would definitely be justified (because you aren't going to make it).
I'd suggest to new owners:
Target 10% arrival at your Supercharger (using the Energy->Trip screen), if conditions are ideal and weather is not dangerous to life, and you like to travel 5-10mph above the speed limit.
If you prefer to travel right at the speed limit, you can probably get away with targeting 7-8% arrival charge for optimal travel time.
However, if you like to avoid any stress, or there is any uncertainty at all about safety, wait the extra couple minutes to add a couple more %.
Pay attention to how you do relative to the projections and adjust your strategy accordingly, but be careful to account for any conditions that may change results (headwind, rain, etc.).