Actually with most carpets it does not matter. They will inhabit the sandbed but only if they can attach to a hard surface where the rock and sand meet. As long as it can bury it's foot and attach securely it should not matter. The only anemone IME
that prefers to be in the sand is the Heteractis aurora
which I have as well. The anemone may be fine in the area you've chosen but it will depend on how much it likes the light and water flow. It might just decide to move to the middle so be prepared...
Do you know the proper species or was it sold simpley as Stichodactyla "sp"