Like others have stated, you will find your own sweet spot One trick some use is to take the resistance of the topper and add 2 to it, then adjust from there. So a 2.2 ohm coil would be a 4.2 starting point, then adjust up or down for your sweet spot. The start low and dial up is also a great way, if it tastes burnt, dial down. Really no magic number, you and I would probably have different settings for the exact same setup.
As for charging, I have set it to 4.2 and turned it off, I have left it on and set to where it was and charged it, didn't seem to make any difference in my case.
Did you look at a 1300 MAH Vision Spinner, about an inch taller than my 650 MAH Joyetech ego, but about half again as thick. Makes a good on the go device as it is not super long and the charge will last me for days.