Waste gate closed, flapper is 22.0mm diameter: Waste gate open, OEM port is 17.0mm diameter/10.0mm deep: Not that much room for error. Thinking I'll open the hole's exit diameter up from 17mm to 19mm. Leaving a 1.5mm seat/seal around the perimeter. Then taper/funnel the inside edges and smooth off the exit area a bit to maximise air flow.