Did you find this method yet?viewtopic.php?f=23&t=20676&start=20#p494600
It was recommended to me in this thread:viewtopic.php?f=9&t=23983&p=495077#p495077
To try this, you need to go on a fairly long test drive. Clear the trip computer and the Energy History on the Leaf's displays, so you can use those as a backup or sanity check. Turn the car off completely then back on. This should reset the Trip information on LeafSpy (found in Summary screen: keep touching the text box just above the left end of the colorful SOC/GIDs/DTE display across the middle of the window until you have cycled to the Trip-Miles-mi/kWh display to find this). If the Trip data says 0.0 miles, note the SOC% on the car's display and take a screenshot of the LeafSpy "summary" screen (and all the others, just to be completist).
Then start driving 10-20 miles and try NOT to use regeneration: Turn Eco mode off, drive in D not B mode, and avoid braking, so the energy all comes from the battery *once* and isn't recycled into the battery then taken out again. Don't freak out about a little regen, just try not to do a lot of it. Driving on a freeway in cruise control at a little less than the speed of other traffic (so you don't have to brake or lift off the "gas" pedal) would be ideal. Then stop and record the SOC, the miles and the mi/kWh shown for the Trip in LeafSpy (and all the other screens with screenshots, just to be sure), and the SOC, mileage and miles/kWh shown on the car's display. They should be close but may not agree exactly.
Now you can calculate the battery capacity: Divide the miles by the miles/kWh. That gives the number of kWH used in your drive. Then, subtract the ending SOC expressed as a decimal (i.e. 0.534, not 53.4%) from the starting SOC. Divide the calculated kWh by the difference in the SOCs and you should get a number around 20. That's your total calculated usable battery capacity in kWh. If it's < 18 that might be bad news.
Multiply that number by the mi/kWh you expect for your commute (about 5 in the city, about 4 at freeway speed, as shown across the top of the range chart) and you get your miles of range. Better add 10 or 20% for comfort, if possible Then multiply that range requirement by how much you expect your battery to degrade in the number of years you need the car to last (say, to 80% of it's current capacity, i.e. 0.80; I'm just making that number up...hopefully that would take quite a few years. 3? 5? 8? I have no idea, but probably not 8 years!) and see if that meets your needs