I usually wait until I get to 25%. Don't forget that there is a heater in the tank that takes up some of the room. I've had to replace mine in JULY. I didn't think I needed a heater when it was 102* during the day. Car went into limp mode. To speed were listed at :55mph for 99 miles, 45 for 25 miles, 35 for 25 miles, 25 for 25 miles.70% of 3.8 Gal (if that’s the true DEF capacity) = 2.66 Gal. So the whole container should theoretically fit when level reads 30% (assuming the reading is accurate).
@sledstorm1 offers sound advice, run it down to 20% just to account for subtle inaccuracies. Even at 20% remaining you’ve still got many miles before the dreaded countdown to death.