In the system menu of the DX7 there's a function called THROTTLE RECOVERY

Set it to ACT or active for the model you're working on.

Then follow the excellent post by Ram3500

Now, when you land you hold the trim down until the engine shuts down. When you let go of the trim give it 1 push upwards. That will set your throttle trim back to its original position so you get the same idle setting every time w/o twiddling with the trim to get a good idle.