In that calculator, you choose all of the ingredients. My method (LP) chooses the ingredients such that the cost is minimized. Also, in making fertilizers for container plants or hydroponics, the ingredients need to be determined for all of the nutritional elements, e.g., Ca, S, Mg, Fe, Mn, Zn, Mo, B, Cl, in addition to N, P, K. LP can easily handle all of this.
It may interest you to know that LP is commonly used to determine the ingredients for making metal alloys by combining scrap metals of different alloys with pure elements as ingredients. This is because scrap metals are much cheaper than pure elements. The fertilizers that you have in inventory, that you otherwise wouldn't use, are analogous to scrap metals. You want to combine them with other ingredients at minimum cost such that the mixture can be used or sold.