Looking at an ESV reading of a pack isn't really the way to tell how much time is left in the pack. About the only value of that reading is to discover when the pack has dropped too low to fly.

The NiCd packs provide good power up until their last 15-10%. Then they show voltage UNDER their rating. When they go, they go fast. Until then, they show good. They show good with 75% remaining and with 20% remaining. So their showing a good voltage only means they haven't obviously given out. It shows nothing about how much is left.

What we have to do with our packs is test and record and figure, in order to fly safely. You have to have a charger that will tell you how many mAh it just put into your used pack. Without that info, you're pretty much in the dark. And your ESV readings will be all you got, and all they'll give you is a warning that it's already time to quit.

Go fly a fully charged pack. Fly 3 or 4 "normal" flights.
Put the battery on a charger and fill it up. See how much capacity went into that fillup charge.
Divide that capacity by the number of normal flights you flew.
Now you've got an idea how many you could have flown.

Lets say you have a 600mAh pack. Charge it to 100%.
Go fly 3 flights.
If your charger says it took 300mAh to bring it back to 100%, then you could have flown 6 flights total, but the last one would have probably ended with the "lawn dart" maneuver. So your pack would be somewhat safe for 5 normal flights and nicely safe for 4 flights a session. Had you looked at your ESV after the first flight of that example, your instructor would have told you to pack it in. Had you looked at the ESV after every one of your 6 flights, the last look may or may not have warned you off that 6th flight.

ESVs aren't really great insurance. Good insurance. But not enough. Test and figure and you'll be lots safer.