The first, and most important, point to make is that the figures we have arrived at are an educated guess. There is a statistical basis behind every one of them, but they provide a rough guide only.
Having said that, the figures do provide an interesting picture of the scope of catering in the public sector.
When you add all the pupils, patients, prisoners, students, care home residents and service personnel in the UK together you get about 15m people.
That means roughly 25% of the population, or one in every four people, is fed in the public sector.
Most of these (9.7m), are at school, while 4.5m are students in further or higher education and 536,000 are in care homes.
Take into account uptake and trading days, and this translates into about 2.1bn meals served each year in the public sector, of which 1.5bn are main meals.
Half of this is accounted for by schools, while the care sector delivers a hefty 576m because it serves its residents three meals a day every day of the year.
Our calculation is that 690m of the total number of meals served will contain meat, about 46bn grammes of it every year.
If the public sector wants to see the impact of a move such as the 20% cut in meat served, then it needs a starting point like this.
But it is important to remember that the foodservice industry is under-researched, so we had to comb through a range of Government reports and other data cherry-pick the most relevant bits.
Often the real figure can only be determined indirectly from something closely related. The most obvious example is our number for hospital meals, which was derived from statistics on NHS bed numbers.
Assumptions have had to be made, too. Take, for example, the amount of meat served in a meal. This differs widely from schools to hospitals to the military, the caterers in each of these sectors operating to menus that conform to very different nutritional requirements and portion sizes.