How to calculate the minimun of Average Fixed Cost knowing the Average Fixed Cost
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.