There used to be a formula I knew of for RC circuits where you could plug in the starting voltage or current, the ending voltage or current, and the amount of time in between, and project exactly what the voltage or current will be after exactly x amount time has passed. I know it rises/drops to 63% within the first time constant. But where's the formula for the other times? Also would like to have the vise versa: i.e. plug in the voltage or current and find the time.