The Voltmeter should be wired so that it turns off when the power or key switch is off, otherwise it will drain the battery pack when the vehicle is not being used. If the controller has an indicator or power light connector it should be attached there, otherwise its positive lead could could be connected to the power or key switch's output wire.
Here are the standard low Voltage thresholds for sealed lead acid battery packs:
Most electric scooter, bike, and go kart speed controllers are engineered to shut down once the battery pack Voltage reaches these low Voltage threshold levels. However some speed controllers are designed for a range of different battery pack Voltages so they do not have low Voltage protection built into them. For these types of controllers without low Voltage protection the low Voltage threshold levels listed above should be used to determine when to stop using the vehicle so the batteries do not become damaged.
Please let us know if you have any questions.