How much should one eat when following the not-eating-after-noon precept? In my understanding, there are a few ways to follow the not-eating-after-noon precept. Here are what I know of:


Two meals per day + juice/sugar water: breakfast and lunch, juice or sugar water for dinner.
Two meals per day: breakfast and lunch, nothing but plain water after noon.
One meal per day: lunch only. Plain water only before and after lunch.


I am a layperson working in an air-conditioned office. In my experience of method 1, I can do with normal amount of breakfast and lunch. But for method 2, sometimes I need more amount of lunch, say 1.5 times of a normal lunch. Never did try method 3.

I wish those who have experience can share their thought on the appropriate amount of food, and also the ways of following it. I believe Buddha had suggested to have moderate amount of food, but how much is considered as moderate?
It will depend on the amount of physical work one does, but since you've said you work in an office, I would say one meal a day is enough for maintaining the body healthy if you have no digestive problems. I started having one normal-sized meal per day 2 years ago and I've had no problems. I'd recommend seeing what fits for you by making gradual adjustments.  

Bear in mind that the following quotes are directed at monks but, depending on one's health &amp; working conditions, laymen are also capable of following the advice:


  “And how does a bhikkhu observe moderation in eating? Here, reflecting
  carefully, a bhikkhu consumes food neither for amusement nor for
  intoxication nor for the sake of physical beauty and attractiveness,
  but only for the support and maintenance of this body, for avoiding
  harm, and for assisting the spiritual life, considering: ‘Thus I shall
  terminate the old feeling and not arouse a new feeling, and I shall be
  healthy and blameless and dwell at ease.’ It is in this way that a
  bhikkhu observes moderation in eating.
  -AN 4.37, Non-decline (Aparihāniya-suttaṃ)  





  “Bhikkhus, I eat at a single session. By so doing, I am free from
  illness and affliction, and I enjoy lightness, strength, and a
  comfortable abiding. Come, bhikkhus, eat at a single session. By so
  doing, you too will be free from illness and affliction, and you will
  enjoy lightness, strength, and a comfortable abiding.”
  -MN 65, To Bhaddāli (Bhaddāli-suttaṃ)  





  And how, bhikkhus, should the nutriment edible food be seen? Suppose a
  couple, husband and wife, had taken limited provisions and were
  traveling through a desert. They have with them their only son, dear
  and beloved. Then, in the middle of the desert, their limited
  provisions would be used up and exhausted, while the rest of the
  desert remains to be crossed. The husband and wife would think: ‘Our
  limited provisions have been used up and exhausted, while the rest of
  this desert remains to be crossed. Let us kill our only son, dear and
  beloved, and prepare dried and spiced meat. By eating our son’s flesh
  we can cross the rest of this desert. Let not all three of us perish!’
  
  “Then, bhikkhus, the husband and wife would kill their only son, dear
  and beloved, prepare dried and spiced meat, and by eating their son’s
  flesh they would cross the rest of the desert. While they are eating
  their son’s flesh, they would beat their breasts and cry: ‘Where are
  you, our only son? Where are you, our only son?’
  
  “What do you think, bhikkhus? Would they eat that food for amusement
  or for enjoyment or for the sake of physical beauty and
  attractiveness?”
  
  “No, venerable sir.”
  
  “Wouldn’t they eat that food only for the sake of crossing the
  desert?”
  
  “Yes, venerable sir.”
  
  “It is in such a way, bhikkhus, that I say the nutriment edible food
  should be seen. When the nutriment edible food is fully understood,
  lust for the five cords of sensual pleasure is fully understood. When
  lust for the five cords of sensual pleasure is fully understood, there
  is no fetter bound by which a noble disciple might come back again to
  this world.
  -SN 12.64, Son's Flesh (Puttamaṃsūpama-suttaṃ)