This section always reminds me of the old child's riddle:
Answer: MAKE APPLESAUCE !!
When you return to your quarters, right away you should close your eyes and clearly envision the number of individual places in the [sangha] hall; the number of monks in the individual quarters of retired minor officers, retired senior officers, and the like; how many individual monks there are in the infirmary, geriatric quarters, temporary quarters, and so on; the number of wandering monks registered in the guest quarters; and the number of people in subtemples. After carefully calculating in this way, if you have the slightest uncertainty, ask the hall manager in question, or the quarters prefect, quarters chief, or quarters head seat of the various quarters and eliminate your doubts.
Now carefully calculate: for every grain of rice to be eaten, one grain must be supplied. If a single grain of rice is divided, then you will have two half-grains of rice. Three tenths, four tenths; one half, two halves. If you supply two half-grains of rice, you will make a single whole grain. [You must be able to see clearly how much of a surplus will be created if you add one unit of rice, or whether there will be enough if you take away one unit].
From: Tenzo Kyokun - Instructions for the Cook by Eihei Dogen - Translated by Griffith Foulk [with portion from Uchiyama]