Fly for "X" time and cycle the battery down is probably the only method to know how much battery capacity is remaining. Voltage is only a very course indicator of battery capacity remaining, which is why almost everyone stops flying at 4.8 volts.

The starting voltage on a fresh charged pack will be about 5.45 volts or so. The first flight will knock the voltage down to about 5.28 volts. The second flight will reduce the voltage to about 5.18. The third flight will reduce the voltage to 5.12. The fourth to 5.04 volts and the fifth to 4.86.

Cycle the pack down when you get home and hope to have 250ma or more left in a 1100ma pack. Each battery pack and each airplane and your numbers will be a little different.

What you don't want is only 20ma or so in the pack when you get home. Twice over the past 20 years packs have had less than 15ma when cycled down at home. The batteries were discarded.

Bill