Meh, I always top them up before a game. All batteries lose a bit of charge in storage. If i read your post right, you're likely using a wall wart since you are trying to calculate charge time. Go buy and use a smart charger. Takes the guessing right out of it. If it needs a full charge cycle, it'll fully charge them up. If they are holding a decent charge, it'll top them up and drop to a trickle charge so it doesn't over charge.
Cliff notes: use a smart charger