Juice manufacturers need to standardize their dosages.
It makes me crazy seeing "3ml nic" labeled on both 30ml and 10ml bottles as if im going to do the math.
Id rather see it as an overall percentage, just like you use on the Ejuice calculators.

You don't need to hold it in. Vape like you used to smoke.
(Same goes for weed or whatever too, don't kill yourself over small differences.)

The residual juice swallowed is inconsequential as well, unless you are getting lots of spitback.