Oh-oh! Beware Horace: as WotEver alluded to, if it's something along the lines of the NASA or Victron BMV which use a shunt to measure current, these things often say 100% long before the batteries really are, so it can trick you into thinking your batteries are full when they're not.
If you can also read Amps on your meter, I believe it's better to monitor Charging Current as you charge and use tail current to assess when your batteries have stopped taking further charge, i.e. because they're full. The Amps will drop off as the batteries fill, so watch for a steady lowest tail current figure. Two percent of your battery capacity is often quoted (e.g. 400 ah bank capacity would suggest an 8 amp figure at fully charged), but that's a bit high for me. I'd be looking closer to a couple of amps, but the important thing is that it steadies out.
If you can, I'd also recommend you monitor voltage whilst discharging - you can find a resting voltage based table through Google which will give you another indication of your state of charge. Amp hours used shouldn't be far off, but use your % state of charge reading only as a rough guide and don't rely on it, particularly as an indication of when to stop charging. Don't be overly optimistic about the battery capacity figure you give the meter either - the cheaper the battery perhaps the less likely it is to meet what's written on the label and it will be diminishing.
Battery monitoring is contentious and difficult to get your head around, but the above is my quick and dirty thoughts on it in case it helps.