I use the test readings about 2hr after the evening meal, but as I take a walk most evening it may not just depend on the meal.
Say the readings from the last 7 days are 5.5, 6.2, 7.3, 5.3, 4.9, 5.3, 6.0
I then sort them into order to give 7.3, 6.2, 6.0, 5.5, 5.3, 5.3, 4.9
Then take the middle number to get 5.5
This give a easy to work out number, that excludes "invalid" readings etc. Remember that if two reading are different by less then 10% it is very possible the BG is the same. (Even a 20% one off difference could just be the meter.)
(It very possible that by keeping all your reading under 7.0, your average reading is a lot less.)