My personal theory: there are so many factors that would contribute to what MY body needs today that my personal RDA would change from day to day, sometimes moment to moment ... stress ... exposure to toxins ... diet ... amount of exercise ... exposure to sunlight.
Those of us drinking distilled water with all the rock minerals removed would have a different iron level need than a person that's simply made a better choice and still using something like a PUR filter...(even tho I believe that the minerals they are getting are accumulating and not being assimilated).
There could be safe limits, but when getting them from totally food sources????? Be a brave self-experimenter ... eat enough sorghum to cause a mineral imbalance and report back to us......