Are you sure you don't just need "food supply"?
In America the food supply is over abundant, leading to waste.
Most agrarian communities reach a saturation point where they produce a food supply that precisely feeds the community.
The food supply in deserts tends to be on the brink of catastrophic collapse.
The actual indicator would be the supply, while it is the degree of the indicator that would indicate the level of supply: under, over or exactly sufficient; high food supply, low food supply, sufficient food supply.
If you're looking for a specific label for the sufficient point though, and would like to name the indicator after that state, then I think you have two choices: either stick with trying to turn your adjective into a noun, or add a noun commonly used to name indicators. For instance:
- Food Supply Sufficiency
- Food Supply Adequacy
- Sufficient Food Supply Index
- Adequate Food Supply Measure
I have to confess that in most cases, this one included, I react poorly to that sort of flowery academic language. I have a special disdain for indicators that attempt to peg themselves to arguably arbitrary scales. Your description already matches the term "food supply" pretty precisely, you may not want to coin another.
May I suggest instead that you own your formulation of it, i.e. "Quatermain's measure/formula/method/indicator for/of Food Supply"