I'd expect the power used by the a/c to depend on how much work it is actually doing - so if it were always doing the same work, then the impact on mpg should be a fixed mpg amount not a fixed percentage of mpg. In my old German car, if you lift the lid you can see the clutch for the a/c compressor cutting in or releasing as and when the thermostat in the car decides it needs more cooling effort. I imagine the FIAT system is similar (?), so if you set the car at 'Low' it should be doing more work than if set at say 20c as it will be doing work more of the time, and so hit the mpg a bit harder.
I tend to leave the a/c on >95% of the time as even in winter it prevents misting up and generally makes the cabin a more pleasant place to be in.
As to the mpg, it varies so much depending on whether the journey is on the flat or hilly, driving style, short vs long et etc. Also, even leaving a window down or roof bars on can punish the mpg...
Thus far, I'm just pleased that the Mk4 is more economical than the Mk3 - less markedly so on short journeys which (unfortunately) I have to do every day on the station run - however it is actually a viable and decent proposition for long journeys (which with the best will in the world, the Mk3 was not). I'm finding that on these it is decently economical as well - probably close to a 330d BMW which will also do low to mid 40's if driven sensibly.
TBH, I think there are so many reasons to love the little Panda 4x4 that I've not really worried about the mpg at all. Lastly, I have tried to run the car on V-Power (or is it V-Thrust Max or some such daft name Shell give their fuels), as I found the old Mk3 was much better for it and so I'm assuming the Mk4 will be as well.
Sorry, a bit of a ramble and not really contributing to this thread; just a few thoughts from a fellow Panda fan