Another good way to Science up this food problem is do a force embark with say 100 dwarves. Then use your current calculation to figure out how much food and drink you calculation says they need. Then use the dfhack points command to give you a ton of points to embark with exactly that much food and drink. Dig in quickly, burrow them all, then seal off from the outside for good. Don't let any migrants in, they can just starve on the surface. Now go about business as usual making legendary rooms and dining halls so deaths won't affect them much.
Don't have any contact with the surface in any way. Do no farming or brewing at all. Just let the dwarves hang out, eat, drink, and then record how many days since the start of your game before food and drink completely ran out. Then you can compare the days they survived to what your calculations say to test it's accuracy.
Might even be worth doing a few science forts. One you keep everyone busy say, mining and then hauling stone back and forth forever. The other fort you let them all just hang out and throw parties. See if the work load changes food/drink consumption and survival time. I assume that in a busy fort it will last longer. Dwarves will be hungry while finishing a job then go eat. But if they are idle they will just go eat right away. So you may buy valuable days by keeping dwarves very busy. Or it may go opposite with jobs reducing the counter for food or drink causing them to consume faster.
You don't even have to wait for them all to die. If they all eat at a fairly constant rate they should finish off all the food and drink and succumb to it around the same time. Possibly leaving one or two insane dwarves left who managed to get the last bite or drink. Just need to know exactly how food lasts.
Edit: A random thought. How's your vermin catcher population? Do you have any pastured on the food stockpiles? You may be losing food to vermin without realizing it.
