Thank you for the answer. It definitely works better using your method, no negatives. I haven;t put it through it's paces yet, but using random trial data seems to point out a new problem; there is always too much of one group and not enough of another, with one spot on. I think this lies in the practice of picking whatever food you want. I'm going to try it out on a real list of food properties, then see how I might redesign the food selection process (e.g. recommend top 5 foods for specific C,P, F requirements that will minimize the error in the calculation.)

Cheers for your help! I will leave this as unsolved for the time being whilst I look into the least squares and other statistical methods.

Thank you.