Personally, I would leave the water flow on at all times, except perhaps when feeding the BTA, or doing water changes.
If the water flow is too great for the BTA, it will move. If you want the BTA in a particular location, reduce water flow there, but prepare for it to move if it still doesn't like the location.
It can take weeks of adjusting water flow so an anemone will be happy in the location you like, and the anemone will move to where it likes it in the tank.
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