Given the Average Fixed Cost, how does one calculate its minimum?
As the quantity of output of a good increases, fixed cost (by definition) remains the same and therefore average fixed cost per unit of output continuously decreases. Therefore the average fixed cost curve does not have a minimum, except in the sense that it is asymptotic to zero.
A much more important concept than average fixed cost is average total cost, that is, the average of the sum of fixed and variable costs. This often has a minimum, but can be continuously decreasing when there are significant economies of scale.