Firstly I do all my readings with mmol/L measurements
I do my tests at 07:00hrs (Or when I first get up) Reading around 7-9mmol/L
13:00hrs (I eat nothing between getting up and this test but drink lots of water) reading about 6-7 mmol/L.
16:00hrs probably have a cheese sandwich or similar on brown bread reading about 6-7mmol/L
20:00hrs (have my main meal about 18:00hrs) reading between 5-8mmol/L.
22:00hrs may have a sandwich on whole meal bread for supper reading between 6-8mmol/L
At bedtime reading usually between 6-8mmol/L
I then average all these readings for the day i.e. add them all up and divide by the number of readings.
I then add all the daily readings together for the week and divide by 7.
As you have said readings will vary as do mine as long as my weekly average is between 6-7mmol/L then I do not worry.
If you go to the diabetes uk website then you will find lots of information. As I said previously you have to take internet information and analyse it closely.
Remember the A1C test is an average over three months so your readings will indeed go up and down.
Hope this helps.