It won't be accurate. It's guesstimating based on injector duty cycles (based on the ecu signals sent to the injectors, not based on whether the injectors are working or not), and as the air-fuel ratios are different on LPG, and the LPG injectors are "trimmed" to a different calibration than the gas (even with the same AFR based on O2, since stoich is different for LPG and gasoline), it'll always be a little off.