The flat discharge curve of nicads is the reason for the loaded digital voltmeter. The loaded digital meter reads in tenths and hundredths of a volt to be able to better estimate the portion of the discharge curve you are on. The discharge curve steepens quickly at both ends of the curve. This means the pack loses voltage quickly when fully charged then maintains a pretty steady voltage until it is almost discharged then loses voltage quickly again until it is dead. The point at the end of the curve where the voltage drops quickly is what you are looking for.