I didn´t work with food samples. I just measure some water samples. Probably the conc. in my samples was very low in comparison with food samples. My calibration curve works from 0,5 to 20 mg/L of N-NH3.
I prepared the calibration curve with ammonium chloride stock solution. Then I used about six dilutions to prepare the calibration curve. The equipment give you the HCl ml but you must calculate the concentrations by yourself (or with the PC) using the calibration curve.
I used HCl 0,05N but you can change it depending of the ammonium range (if you expect higer N concentrations you can increase the HCl conc.). Of course, you must know the HCl factor every time you prepare new solution.
I developed my own method to prepare the receiving solution. I used bromocresol green, methyl red and boric acid. Then I add NaOH slowly until reach the final color. The NaOH improves the balnks readings. Probably you will find some procedure in the operation manual. Anyway, I run a new calibration curve with each new batch of receiving solution.
I hope it helps,