At least I thought it was simple when I started this paper. Of all the studies I waded through, I found only one that appeared to be helpful to the issue of what is the recommended vitamin A level for ferrets. White et al. They estimated the youngsters could have survived 5 weeks on a vitamin A-free diet. One other study seemed interesting and possibly important to me.
Vitamin E facilitates the absorption and conversion of beta-carotene to vitamin A in the ferret.
The researchers do not know why this is happening. So if you are using a food preserved with vitamin E and the predominant source of vitamin A in the food is beta-carotene, the ferret would absorb higher doses of vitamin A than usual. Absent firm numbers for recommended vitamin A levels for ferrets, I am forced to assume that ferrets need approximately the same amount of vitamin A daily as mink. That is 440 IU per pound per day for an adult. I am also willing to assume that commercial feeds are designed to simulate the nutrient levels in natural meat.