I use the following two methods:
1. Assuming the battery holds 80000 Wh of usable energy, divide that number by your lifetime average to get an estimate of total range:
80000 Wh / (257 Wh/mi) = 311 miles.
To get rated range:
80000 Wh / 265 mi = 302 Wh/mi
However, there are some losses that don't get included in the rated range number, so driving at 302 Wh/mi won't get rated range, which is why method #2 is more useful.
2. Note the miles remaining at the end of the charge. During driving, add the miles traveled since last charge to the current miles remaining to see how many "real miles" were gained or lost. Example:
Charged to 210 miles.
Since last charge: 100 miles.
Miles remaining 135.
100 + 135 - 210 = 25 miles gained over rated range.
As long as the number is a gain from rated range, you know that you have at least rated range left. (Doesn't work if there are substantial elevation gains or losses.)