There's no magic in any one time.....just the same time each time.
Obviously, testing @ night and the next AM gives an unclear picture of your Cl consumption. Like the rest of pool mgmt, consistency is the key.
My 11-year old's Summer job is testing for pH and Cl daily. We certainly don't need to do it that often but it gives here a little spending money, a better understanding of pool water chemistry, and more adult beverage time for her doting Dad. She has chosen to test in the AM 'cause it then frees her up all day.
PS - just saw Carl's post and agree completely that you'll get better chlorine usage if you add in the evening.

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