It's pretty normal IMHO. When the tank gets low the pressure inside changes and it will get a bit more leaky. When I use carto tanks I clean them often with a qtip or paper towel. I get anywhere from 2 days to maybe 10 on the average carto (depends on thickness of juice) on the average.
Really like the look of your tank by the way, nice. the holes on your carto look big to me as well. Perhaps since you use 80/20 pg/vg maybe go for a smaller laser punch hole. Nothing to really worry about though.
naw i am about to ditch this tank entirely when i get the money soon i am buying a kayfun 3.1 clone and going to start building my coils.