When myself and @Hanuman was putting the
IFC Aquarium fertiliser calculator together, we was incorporating the users water report as much as we could, the main issue we had (after getting our heads around the carbonate/bicarbonate calculations for kH) was the water companies reports vary so much in what they choose to report on and in what units. So some things was left out as having a box to enter a value of element 'x' and unit 'y' when the user cant find the value/unit would of just ended up with folk asking us too many questions.
Indeed, and there was also another issue to deal with and that is the pseudo hardness (PsH). That's when the measured alkalinity (~Carbonate Hardness - CH) exceeds the Total Hardness (TH) making the common hardness relation go haywire. The standard total hardness equation is:
TH = CH + NCH (Non-Carbonate Hardness)
In the case of a pseudo hardness scenario the equation becomes:
TH + PsH = CH + NCH
Having to deal with that was also making us go crazy and in my opinion was just making things way too complicated for the purpose of this calculator. In fact I doubt that anyone will be using the water report for the purpose of making a fertilizer but at least it is there for whoever wants. The reason being that a water report is not always what we think it is. Numbers are the reflection of tests done on a specific day or period of time. For some locations, those values will vary depending the time of year. If anyone relies on a water report to make a fertilizer I can confidently say that the fertilizer being produced will not be in line with the actual tap water you are using as a reference.
This is why I think a fertilizer needs
not to account for a water report
UNLESS there is a history of the tap water being high or low with a specific element and that the element content in the water does not vary through time. I think in the UK it is the case in certain parts with high Calcium (Ca) contents which stays roughly the same throughout the year. Only in that case using the water report makes sense. Still though, Ca is not an element we usually put in a fertilizer, it is usually used as a remineralizer and front loaded...
This link describes pretty well what water hardness is:
Water Hardness
