How do I get the coverage area for a gallon of enamel paint sprayed on a primed wrought iron fence?

Coverages are fuzzy estimates at best, even with standard surfaces. Take the worst coverage estimate鈥攎aybe for masonry鈥攁nd multiply by the fraction of surface area the sprayer will see. If the bars are wide apart, that might be one-tenth. If there is a lot of dense scrollwork, maybe one-fifth. If it is possible to spray mostly on the iron without much ';blow-by'; maybe multiply by one half.