I don't believe this will work to find the state-of-charge of an eGo Twist-style battery; on VV devices like the eGo Twists, these inline voltmeters display output voltage, which is whatever you have the little dial set to ... the output voltage should be the same (within limits) regardless of the actual charge level of the battery. That's what the buck/boost circuit in these VV devices are for, to maintain the "set" voltage for as long as possible, no matter whether the battery is freshly charged or nearly discharged.
AFAIK, there is no simple way to detect state-of-charge on an eGo Twist-style battery without some specialized meters ... items the average vaper is not likely to have.