First, cut the daily charge down to 1 hour per day. The longer charge time is only making heat. The other thing is you did not say what kind of chemistry your packs are, though I assume they are NiCads.

Finally, measurement of how much charge to top off is not that important. Measurement of remaining capacity is what you want to know. Time to discharge to 4.400 Volts will tell you if you have a weak cell if discharged at 300 to 500 mAh. If you plot Voltage vs time it should be a fairly straight line after the initial surface charge is dumped. When it goes over the knee suddenly, one or more cells have no more current capacity. A very well match pack will go as low as 4.3 Volts before you hit the knee of the discharge curve. If it happens at higher voltages, then the pack is not a healthy pack.