I always check most of my readings with very little chlorine in the water; you learn from experience when that will be. The reason is so that the chlorine doesn't skew the readings.
I always test for combined chlorine when chlorine is close to 0 - it makes it easier to see if there is any combined chlorine. Never test for combined chlorine after you have used non chlorine shock - it will register as combined chlorine, you need to wait a few days for it to be back to normal.
Test for free chlorine 20 minutes after you put the dose in. This gives the chlorine time to work on whatever and if it has dropped significantly - add more.
When checking for PH and alkalinity - check PH first, adjusting for alkalinity may change your PH. My water changes PH a lot when I adjust my alkalinity.
And the biggest thing to remember is to learn that the readings are in ranges - there are no absolute numbers when checking the water.
