Hi Prescott,
Looking at the picture a little more it looks like the throttle plate area is designed more like a ball valve that opens up to a flaired venturi.
Regardless, if the throttle plate position is managed by the ecu, I would determine that position, add one degree of clearance to avoid idle oscillation and cut it there.
ie: if the plate idle position is 17 degrees, I would cut at 18 degrees. That usually comes out to being the thickness of the throttle plate.
One of the guys mentioned that putting the groove at the plate position where the engine is cold works well for him.
Karl Fortner
Tacoma, Washington