Great. When you do have to backwash, remember DE will backwash out and you will need to re-add. I use DE all season long because of the improved clarity.
Now that you you know how much to add, it's easier. If you want to make double sure, you can re-do the initial 1/8, wait, check, 1/8, wait, check. I am a little OCD and did the initial test twice myself.
Sounds like a good plan!
Congrats on the First Communion! Wonderful time of life!
Kim
Looking good! You're definitely almost there!