The capacity information I included in post 26, was already mentioned by ByteMe in posts 6 & 7 of this thread. I couldn't follow his calculations as I didn't earlier understand where his figures were obtained from and as he included calculations on 7 days, I don't understand why that was included.
In thinking about the ratio of mg/L being equal to ppm, that is applicable for the iron content in the water but I'm not certain a ppm conversion will work when calculating capacity in a volume of media ie: 3000 mg per Litre of K-L.
As the confirmed iron content is 7.47 mg per Litre of water and as there are 3.78541 Litres per US gallon, then your iron content is 28.277 mg per US Gallon.
28.277 X 3,600 Gal/day = 101,797 mg/day of iron to remove.
As K-L has a stated capacity of 85,000 mg/cuft, then 101,797 / 85,000 = 1.2 cu/ft of K-L media/day, assuming iron removal only, with no manganese or sulphur also consuming capacity.
As a 1.5 cu/ft filter cannot support a 14.5 GPM flow rate, then a larger system is required to meet/exceed your flow requirements. While more media will provide additional capacity so that daily backwashes may not be technically necessary, you probably would want to backwash daily so as to ensure full iron removal at your full intended GPM flow rate.