On your energy information display page it shows how many kWh you have used. That's the true "gas gauge". You will find that at around 14 to 15 kW have been used you are "empty". If you get higher that means you've been good on Regen.
I agree with Arcanox. My understanding is the kWh Used number on the energy usage screen shows a net calculation of the amount of grid power used from the battery less the amount of regen put back into the battery while driving on grid power. Regen doesn’t influence the fully depleted kWh Used number.
IOW, a fully charged Volt battery contains a fairly consistent amount of usable power each time you fully charge it. How far you can drive using that grid power depends on your driving habits, and the environment and terrain encountered during the driving. Whenever you slow down on level terrain (coasting in D or L or using the paddle or the brake pedal), the regenerative braking system will brake the car by cranking the motor as a generator, recapturing some of the energy used earlier to accelerate the car. (Downhill regen captures gravitational-sourced energy.)
If you’re driving in Electric Mode, that regen will recharge the battery (sometimes while driving down a long hill you may observe the kWh Used number getting smaller), allowing you to re-use that amount of charge to drive Electric Miles.
If you’re driving in Hold mode or with a fully depleted battery, the regenerative braking system is still working, recharging the battery as it brakes the car. In Extended Range Mode, regen battery power is used to drive Gas Miles.
Presenting the kWh Used as a net calculation of grid power used removes the influence of regen on the fully depleted number. Each time you drive far enough to fully deplete the battery, the kWh Used number should remain about the same, representing the amount of grid power you stored in the fully charged battery.