Believe it or not my SWEET SPOT changes throughout the day ! In the morning it is a little lower voltage and then increase's towards mid day and gets stronger at night.
I tend to use lower nic juice at night ( so I am not up all night ! ) and lower it before bed to the morning level.
So you can change the voltage to keep " IN THE ZONE " all day !
Any interest in a DESK TOP version ?
I'm using a test rig right now that has the controls and charger of the VVPV in a desk top box with an LED and modified old passthru that has the switch built into the passthru tube , I plan on putting an LED voltmeter into the box as well ,so you can just read out the voltage setting, adjust the voltage with a control Potentiometer ????

, I might look into making these as well ? It would be a great rig for the Computer guy