It is important to know the state of charge because lead acid batteries are damaged by chronically being discharged below 50%.
It is tricky to measure the % of full charge with a voltmeter. When the battery is charging it will have the voltage of the charger on it, which will be more than 12.6 volts - suggesting it is 100% charged. This awkward situation persists for at least an hour after charging is stopped.
The solution is to install a battery monitor that measures the energy of charging and discharging (amp hrs) and shows the % of full charge all the time.
I have a
Cheap battery monitor that works well. Higher quality ones are easier to install and more reliable.