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