The app has a feature to allow you to see the actual percentage of battery remaining.
Open the app and click on the "3 dots" icon on the upper left.
Navigate to the "Show Battery Levels" option and slide it to the right to turn it on.
When the tabs are connected to the app you will see the battery percentage.
Note: The battery level may not be fully accurate. The true test of battery life is how long they last during normal usage. It should be around 3 hours of continuous usage.