The concentrations of the Standards we supply with our kits are designed to give a good working range with a typical spectrophotometer.
For example, the Ammonium Standard is typically 40 mg/L (ppm); this gives an absorbance of around 0.5 absorbance units. Likewise the Malic Acid Standard is typically around 0.2 g/L and gives an absorbance range of typically 0.5 Abs.
The ideal situation for best accuracy is to try to get your samples diluted so that they are of a similar concentration to the Standard.
For example, if you expected a juice sample from a particular vineyard to have about 200 ppm Ammonia, the best dilution rate would be 1:5 to get the sample concentration to around 40ppm.