The big issue is that Zippos aren't that well sealed, especially the lid which allows fuels to evaporate through the wick and it's this wick to air interface which is the most critical thing as the whole function of the wick is to provide a big surface area for fuel to evaporate and thus burn.
Given that the fill level probably doesn't make a big difference as the wick will suck up fuel by capillary action as long as there is fuel to suck.
Temperature will make a difference as it will have a direct effect on the vapour pressure of the fuel humidity not so much as the air won't get saturated with fuel vapour in the same way as with moisture so they aren't competing as such.
also it is not easy to improve the sealing on a Zippo, at least not around the lid where is really matters as the interface between the lid and the body is fairly thin and there is no easy way to keep the lid firmly in place.
Compare to something like this which has a thread and O-ring.
From and engineering perspective the classic Zippo design is simple and convenient but the price you pay is wasting a bit of fuel through evaporation.
Probably the practical best solution is to carry a small screw-top container with extra fuel.