As a first approximation, if the food production falls to a level of X% of the required calories for the population, your probability of surviving is roughly X%.
...but as a second approximation, post-large-scale nuclear war, if there is only 50% as much food as is required to feed everyone in a major city over a couple months, the ensuing violence and hoarding of the food will likely kill many more people than half. And I’m less sanguine than you about the survival of infrastructure needed to maintain the modern world without major cities, ports, power infrastructure, etc. Sure, there might be enough food overall, but if it’s in a different place than you, and there is no modern communications or transport, that doesn’t matter much. If you already live on a farm, great, but otherwise you’re likely to be in trouble.
...but as a second approximation, post-large-scale nuclear war, if there is only 50% as much food as is required to feed everyone in a major city over a couple months, the ensuing violence and hoarding of the food will likely kill many more people than half. And I’m less sanguine than you about the survival of infrastructure needed to maintain the modern world without major cities, ports, power infrastructure, etc. Sure, there might be enough food overall, but if it’s in a different place than you, and there is no modern communications or transport, that doesn’t matter much. If you already live on a farm, great, but otherwise you’re likely to be in trouble.