The voltage with a small current draw, a couple of amps, will be a good indication of the state of charge.
Unfortunately it won't tell you much when charging the battery. That is when the ammeter is useful. As the battery gets charged, the charging current goes down. As long as the converter, and battery, voltage is at 14.4v the current will decrease as the battery becomes charged. You will soon learn to relate state of charge to charging current. The same applies to the other voltages that the converter supplies.
Of course, the ammeter is more difficult to install as it requires a shunt.
